Easily done, especially in a large group.
Personally I take the prize for paranoid parent, but that's only because I have the dubious benefit of someone trying to take DD1 when they were toddlers, she wasn't even unattended as I had her by the hand. I was, however, dealing with her twin sister and so a woman tried to sneak off with her. Hence, I don't care if someone drops dead in front of me, I'm making sure I have both DDs as I can't trust the OH. But as I said, I'm paranoid. A normal parent would be more relaxed and assume t'other parent has the DC.