Custody schedule is killing my back to school plans

My ex moved everything around last minute and now I don't have the kids during the week we were supposed to go clothes shopping for school. We agreed on a pretty standard 50/50 split but she keeps asking for swaps and I keep saying yes because I don't want to rock the boat. I need to set some boundaries but every time I bring it up it turns into a thing. How do you guys handle this without making it worse for the kids?