I assume you are thinking of some sort of hat or cap for them. Cute idea, but I doubt the characters are allowed wear anything that is not part of the "official" character costume. Rather than your son being disappointed I would suggest some small item (maybe a package of peanuts?) that you could give to the minder and ask them to give them to C&D when they are on a break. Even then they may not be allowed to accept anything from guests.