That is 2.5 gallons a hive, roughly 10lbs per gallon, so that is about 25 lbs of syrup each, roughly 16 lbs of sugar, assuming that is 2:1. (give or take a few pounds!)
It all depends on how much they have. It won't hurt to keep feeding them, but if they have enough stores then it may not be necessary.
If you enjoy feeding them, go for it! If you are cheap like me and hate paying for suger, have your son lift the hives, you need ~100lbs of stores in each (+the hive weight).