Yes I would echo dinged's concerns. Children who have been brought up around polyamory from a young age will generally be far more accepting than teens and pre-teens that haven't. My eldest daughter (in her 20s) has always known and while she doesn't agree with it she respects my decision. I told my younger daughter at 18 or 19 only when she saw this forum up on my computer and she nagged me to tell her what it was all about. She was very upset and I had to downplay the whole thing. In our case there isn't that much going on anyway. I think with her and possibly other teenagers they project their own fears of infidelity and betrayal.
My partner's kids are still adolescent and they don't know. They know about his SO and have even stayed with us when she was staying but they only know her as a good friend. Like you, my partner is afraid of his ex stopping visits due to inappropriate behaviour.