I would probably do two small gifts in case they set it up where each child is holding their gift and handing it to the child or put something fancy on the card for one to carry and just a name tag on the actual gift. I would spend no more than $25 on both gifts. Realizing that both will get the take home bags, eat the food, play the games - sort of the way we treat siblings as in no sibling discount. But the total spent is not really more than if just one was going but I wouldn't skimp if that makes sense.