Most every family sharing type system is somehow clunky. I sign my kids up as adults for most things as child controls are never clear until you get into the app and realize nothing works, or inexplicably fine content is unavailable. (With the note that my young kids don't use electronics monitored anyhow..)

Having said all that, Apple's family sharing has been the easiest for me to use of all of them at this point.

As for the story I'm wondering if the court can help, no it's not fun to involve them, but it seems like they possibly could order the ex to release the kids accounts.

If not and the ex is unreasonable, then maybe it is time for new accounts, even as disruptive as that might be. Even safety wise that might be a good choice.