Schools have the wear shorts or leotards rule for the girls so that when they are going up the ladders etc on the playground equipment they aren't flashing to the boys below. It is a rule that could be put into your daycare handbook. Since it will be a good one to have with potty training/newly trained girls in that accidents will happen and the shorts will absorb some of it. That would be a way to put it to your friend that you have this rule and in fairness her daughter should adhere to it when she comes over.