My siblings and I frequently took friends along on out of state vacations.  My parents would get a signed document from the friend's parents that gave permission for the friend to travel with them and gave them consent to get medical or dental care in the event of an emergency. The document included contact information for the parents, insurance information, blood type and relevant health information, like allergies or chronic conditions.   If there's any question about whether you have authority to travel with SD or obtain health care for her, I'd get something similar from BM, too.

We take the kids out of state all the time. It’s not even if our order unless it’s an extended time because BM lives right next to the boarder. Even longer trips we didn’t do anything extra because he’s their father.

If BM does ANYTHING fishy I’d get a lawyer to handle that crap quick. You can always talk to the cops about her knowingly filing a false police report if she tries anything through them. They don’t like that very much. I’ve always made very sure that anything big we have planned is on his time and that way if she every causes real issues we can sue for reimbursement. Even if that doesn’t work a family court judge won’t like that her acting a fool caused such big issues.

So anyways unless there’s really something unique your partner shouldn’t need anything special for his kids because he is their parent and there for has all rights.

Now the other kids you want some sort of written proof that you have the parents’ consent to take with you. It doesn’t have to be anything big… Think about the permission forms you signed when your kids go on field trips. Basically says you’re taking them X place to do X kind of stuff. You’re planning to be back by X day. Contacts for you and them. Recognize that without getting paperwork for temporary guardianship you can’t sign any sort of release forms for the friend’s kids. Bring their issuance information but the hospital will still contact their parents for permission. Really the only thing you want is to protect yourself from a lawsuit if anything happens. You want to make sure that you can show that they consented to you having the kids and knew what was going on so they can’t try saying “we didn’t know you were taking them to a water park” or whatever if something happens.

I guess we got super lucky.. nothing in the CO required we get anything from the EX.  And... we just flew by the seat of our pants with the kids' friends... but we also had a rule that we wouldn't take a friend until they were old enough to be fairly responsible... so I think it was maybe 14 or 15 yo before they had friends on trips with us.  Oh... get money from the other parent because it is expensive to sport the other kids.