Make waves because children are only young ONCE and a parent needs to be sure they are learning what they are supposed to be learning.
Even as a SM, I made waves.....and I know that EX#3's EX is glad he is now my EX and doesn't have to deal with a "motivated" EX anymore.
I would get totally smart on homeschooling rules and requirements for the county in which the children lived. And then give Dad the ammunition he needed to correct the situation. After that it's up to him.
Again though, like you acknowledge, they are
DH's children. So you gotta follow his lead......