S.H.
He's doing it because it is convenient for him.
As you know.
My son is 4. My daughter is 8. (she is not developing yet), but they take showers/baths together and sometimes not.
But my daughter does not mind. Yet.
When they do take separate baths, it is not a big deal. For the kids.
Your Ex, should get the book "The Care & Keeping Of You" by the American Girl book series.
Or you get it for him.
It would be good for you too.
It is written for the child.. .to read and with the parent.
It is for this age girls.
I have one already, for my Daughter.
The American Girl series, has great books for girls in this age spectrum.
Just look online, about the reviews etc. Such as on Amazon.
From here on out, from 8 years old... a girl goes through many changes physically and emotionally.
It would behoove him to read books like this.
From 9-12 years old, this is the "Tween" ages. ie: Pre-Teen.
There is a huge difference, between a 4 year old and an 8 year old.
He best learn about this.
Also- Teach your girl, to speak up. From here on out, girls need 'boundaries' per their bodies/emotions, due to their age development.
Your Ex, should KNOW this.
all the best,
Susan