S.S.
There is certainly no easy answer for this situation. If it was me, I would call them out on the behavior. First, they might not have a clue that they're even doing it (or that it's so obvious). If they choose to continue the behavior, ask that they no longer send/bring gifts unless every child is treated equally. Or even better, you could suggest that the money they are spending on individual gifts be put into family gifts that everyone can enjoy: zoo memberships, gift cards for dinner, movies, book stores, amusement parks, etc.
Good Luck!