We just returned and remember United announcing that everyone 18 and over must have an ID. I never saw them ask for any document for the kids. When we take our 8 year old grandson, he has his own MVA Picture ID. This is something he carries with him on every trip to WDW with us. The airline folks are always surprised to see him with his own ID, plus, since he's an AP holder, it's always good in case there is any question. It only cost $10 at our MVA and makes him feel special. Also, since we're not his parents, we always prefer that he have some type of identification. I think birth certificates should be fine and would be peace of mind for you.