Hmm - I have seen children with contact allergies before where if the food was on the skin they broke out in hives rash however they could consume it without issues - usually acidic things like oranges, salad dressing, ketchup and so forth ...milk would be a new one for me???

Is they child's skin sensitive at any other time?

Personally I agree being cautious and not serving milk in the meantime is reasonable considering the risk should he have a serious reaction while in the group care