So OP, you didn't answer my question, which was:
"Why can't you go too? You don't need to be 'othered'. If it's just that you don't want to well fine but then don't stop your child/ren from going"
Surely, if you feel uncomfy with your child going into old territory without you, then you could go too?
I'm not trying to be combative though i can see that you've taken my comments that way. I really don't understand what the big issue is. You can all go, fine, even if you have to wriggle around a collection issue. You don't want to go? Also fine, what's the problem with your child going with your DH? Is it that you have a concern that they won't be happy/well looked after/doing activities to please them as well as their siblings? Or is it more a 'you' thing, where you feel territorial over your child being 'there'? Your child is also your DHs child, equally.
I think you need to try being honest with yourself. If it's a concern about your child, then discuss with DH and try to resolve so that you feel happy. If it's you feeling like you don't want 'your' child 'there' for no good reason except you don't like it then we will have to agree to disagree because I don't think that's right and the only people who suffer for that is the children.
I think allowing and encouraging a sibling relationship to flourish is a very valid need for children and the DH who is encouraging this is sensible. No, not all siblings (even full siblings!) relationships are as nice as the one I've got with mine even more so in blended families with steps and halfs etc. But if you can help to make it so, why wouldn't you?
As a side note, your DH could pierce your joint child's ears without your permission because he has equal say. But he'd be a total arse to do that without checking with you first and I completely agree that if you weren't happy (I wouldn't be either!) then he shouldn't be doing it!