Ridiculous situation to be in. Have this issue in my own family, back and forth about the "clothes", who bought what, bring it back, want it back, etc. etc.
Don't take this wrong, totally understand your point, you purchases the uniforms so that is your right to want them back and not have the X keep them. If she is doing this deliberately then she is mean.
But at the end of the day you know who suffers from the back and forth on clothes? the children. Maybe your kids are not and that would be great. If the kids are caught in the middle of the clothes and who has them it just creates stress for them...divorce is hard enough and at the end of the day it is just clothes.
If the kids need their uniforms for the next day, then just maybe they can put them in their back packs when their Dad picks them up so they have them for the next day. its a tough situation and so stupid to have to deal with.