Not sure if you mean birthday parties for your BF's actual children, or if you mean parties for kids of their social circle (and both Mom and Dad are attending together with the kids as if still intact couple and family).

He is capable of having birthday celebrations with his own children minus their mother. Not uncommon at all for both parents to have individual celebrations. 

Mom and Dad both can and will be attending kid's sports events. But why do they meet up together at event. These people are either getting divorced or they are not. The sooner they adjust to not acting as a married socializing couple at the kid's school/sports events the faster the children adjust to Dad and Mom no longer being a couple. 

Yes, they are both the children's parents, and always will be. Yes, they are both the children's families and always will be. But when they are going through a divorce they cease being a couple. And yes, it may mean both parents may have to figure out how they still socialize with the adult friends they have acquired together over the years. 

After the 'official' announcement that Dad and you are a 'couple', what are the plans for handling these joint events (school and sports)? Does it mean Dad and you both sit with Mom (how do you feel about that?). Does it mean Mom sits elsewhere? 

Discuss the issues now. Never a good idea to wait and discuss it afterwards. You'd not want to be introduced and announced 'a couple' to the children if Dad isn't going to treat you as a partner and take your feelings into consideration. 

How will his children's birthdays be handled? Joint parties? Individual with each parent doing their own thing? If joint parties, will you be invited an participate or be the third wheel hiding in a corner full of jealousy and resentment? 

How long has he been divorced and how long have they been living apart physically?  If more than a year, I would assume all DC's classmates know.   If it is going to bother you that they still talk or even sit together, you need to talk to him, but he may not agree with you.  

If your BF's ex is sane and they are able to talk and co-parent, be grateful. Don't be insecure about what the other parents think, who cares?  If you are lucky, you will not have to deal with the toxic crap many of us deal with due to crazy exes. 

I have a friend who sits with her ex-husband and his new wife at games sometimes.  Everyone gets along great. It's part of healthy co-parenting, provided the two exes have good boundaries (ie, they don't talk about their new relationships or other personal stuff).

Your gut is right. They are playing family. Now his kids will have memories of these said family events where they know their parents are “together” (kid logic) and I’m sure they’ll have pictures to prove their memories are correct that their dad was “dating” you while still with mom. Sure as adults we think: oh no, they’ll understand one day that their parents where just co parenting—-NOPE! Children of Divorce are a different beast. They will see you as the interlopers, the mistress, the one that distracted daddy while he and mommy were working things out.....Never play house with a man who is still playing games with his Ex especially if they’re using “it’s for the kids” excuse. Because that right there will tell you where you will always fall on the totem pole.

Kids -> BM (for the kid, of course) -> you 

No two situations are the same.  Some people have this great kumbaya relationship with their ex and new partner where all are happy together.  There's no arbitrary right or wrong.  When that works for all - so be it.

But you as an individual have a right to decide what works for YOU.  When I met my DH he and BM had been divorced for 20+ years.  She had remarried and her DH did not want DH in their lives (yeah him).  DH moved to the city where BM and I lived when he moved in with me.  I did not know BM but she lived less than a mile from me.  One day DH stopped by to say hello to her since he was living so close by - I went ballistic.  That was NOT ACCEPTABLE  to me.

I know that that there was no way on earth that DH and BM would ever reconnect romantically under ANY circumstances.  But it was absolutely not acceptable for my DH to have a close friendship with a woman he had been married to.  Their children were long grown.  I realize that many will think I was wrong - but tough sh*t - I get to decide what is okay in my life.

My decision was drop that crap or begone.  My life and I get to choose how I share it.  No one else gets a vote.

I understand how you feel. I didn't watch but knew my husband posed for countless photos at my SD's wedding with his ex and their family as if I had never even happened. I WAS right there and it was STILL unnerving!

But even you wrote that it shouldn't matter what others think. And it shouldn't. Largely, you are allowing your mind to go places it shouldn't. You're allowing a roadblock that doesn't have to be.

At the same time, your skids will get married too, and between now and then, there will be many situations that will pave the way for the same feelings you have right now (graduations, birthdays, grandkids, etc.). Be realistic -- some of this is unavoidable if you choose someone with kids and an ex. It's up to you to get your head right and keep it at bay or you will be miserable.

Be sure he is worth the added insecurity exes and skids bring to a relationship. Be sure you're up for the challenge because it is devastating when you let your mind go there.

I can't say I forecasted this devastation when I married. But by the time SD's wedding rolled around many years later, I knew the drill. I knew when I agreed to go to SD's wedding that the photo scenario would happen -- it was unavoidable because of course SD would want those photos -- and without me. I planned ahead, imagined how I'd feel, figured out my strategy and got through it. It was painful, but it was momentary.

If you're really, really lucky, his kids will adore you and maybe you'll even achieve friendly terms with his ex. Given what I read of exes on this site, it's probably a blessing that he does get along with his ex.

Either way, it's never good to put too much attention on what others think. Be confident. Be prepared. Be master of your mind and don't let it get in the way of your happiness...and likewise, don't let it fool you into thinking love will conquer all. It's very difficult, but it's manageable if you're grounded in reality.

To discusses these issues.  To the point to get it down in writing. On how these events will be handle.  As the new wife. He has to end playing “ Happy Family “ with his EX.  He is giving false hope to his kids of getting back together with the EX.  Once kids figure out that you are there, all hell is going to break lose. 

Make up a list of events, Birthday party’s, holidays, sport and school events. Vacation,  what is going to happen on each of these.  Are you doing your own Birthday party’s, or with EX. Ect.  Remember as a new family you will want to make new memories, what does not include his EX.  When you have your own chrildren, what will happen ?  You will want Christmas at home what does he want ?  Please don’t wait. He may not be the one for you ?

yeah, be grateful. I mean my SO had to get a court order saying that his ex wasn't allowed to be on school premises at the same time as him (one school pickup every two weeks) because she'd had screaming fits and attacked his car (which kids were in at time) in front of all the other parents. A few years later she pulls him into a social services meeting because she wants the order lifted. She has calmed down a bit, but not entirely. 

Also be grateful, because frankly all those kid events are DULL AS DISHWATER (with rare exceptions). And if you intend to have kids yourself you'll be sitting through plenty of those and some of the stepkid ones will be unavoidable, too.

You need to get over what other people think and basically if you're worred about this there will be a lot of avoidable self-torture over the years as you worry about it. They each have a right to be at these events. Most of the other parents either will know or don't care. 

However - one red flag I see here is that your boyfriend hasn't mentioned you to the kids yet? I don't think he should be surprising them with you just showing up. These things need to be eased into. 

When I told my son about my SO - the only guy I'd mentioned to him since his dad and I split - he cried. He literally burst into tears. It was heartbreaking. He didn't know my SO, so it wasn't personal. But he needed time to adjust himself to the idea of it before carrying those ideas into an actual face to face meeting - which was about a month or six weeks later. By then he was more open to the idea and willing to meet SO as a person, though he still wasn't thrilled about it. 

Over a year later and things are really good. But this is a massive change for a kid and one they have no choice or control over.