I would err on the side of being at the initial consult with the therapist.  After that the therapist should map out a process to work the Skid, the BioParents and the Sparents as well as eventually any other kids into the sequence of therapy.

When my XW and I engaged a therapist it was a well defined progressive process.  My priority was to address the lack of intimacy in our marriage. In the 18mos we had been married at the point I finally got her to agree to seeing a therapist together we had been intimate maybe 4 times.  As newlyweds both in our early to mid 20s (She was 20 when we married, i was 23).  We started with the therapist when I was 25 and she was 22.  The Doc tuned me up in a hurry letting me know that we would get to the topic of intimacy but had a lot of work to do before we got there.

So... we went through each of us and our individual relationships with our own family.  The Doc asked for a session with each of us and our parents then each of us and our own sibs.  Then we worked on she and I and how we worked within our marriage.  Through all of that there was an improvement in  intimacy even before we actually covered intimacy in therapy.  After many months of working with the therapist to progress through her progressive plan for us we walked in to session to hear "Now lets start the discussion of intimacy  and sex in your relationship."  My XW stood up, announced that she did not have a proble with sex, walked out of session and never returned.  2.5 months later she moved out of the home we had bought a few weeks before the therapy session that she walked out of.  

I kept seeing the therapist for a total of 5 more months on my own until the Doc informed me that "When you walked in here 10 months ago I would have never guessed that inside that sullen upset man that there was a young man with a child like zest for life. You can of course keep coming to session but you don't need me any more. Get out there, live your life, and be happy."

That amazing woman helped me reconnect with the man and person I like being.  Reconnected with that confidence, I started dating the evenign of the day my then STBXW moved out.   The next day I had the locks rekeyed, and the first weekend after my XW moved out she started banging on the door when her key no longer worked until I answered the door with a towel around my waist and a beautiful blonde wrapped on a sheet standing behind me.  That was fun. Let me tell ya.

From my last session with the Doc I had total confidence in dealing with my XW, the divorce process and her wierd "we can date if you want" crap.

My point is, do not let anyone push you out of fighting for your marriage or any therapy that may impact your marriage. As a SParent  you have a unique and absolutely unclouded view of the actual behaviors, traits and character of your Skid, a particularly cogent view of BM and her crap as well as a very close and personal view of your DH's strengths, weakness, successes and failures as a partner and a parent.  The therapist should seek your input and you should participate fully in the process.

IMHO of course.

Take care of you.

Both parents should be involved on an ongoing basis. Child therapy is not about a kid meeting alone with a therapist - often the parents are part of the problem, and always, they are part of the solution.

Stepparents should go if invited by the therapist. Otherwise, just stay out.