My ex has my kids shower with his other kids....advice?

Question on what to do when your 6 year old daughter tells you that when their at their dads for the weekend that her and her 5 year old brother have to shower with other kids example my daughter told me that she has to take a shower with a six-year-old girl and her five years old Brother has to shower with a 9 year old girl?!? And when she tells her dad that she does not want to and wants to call me (mom) he tells her no and to deal with it!?! What would other moms suggest to do when it’s only on the weekends