Your poor kids. What a cruel thing for their father to do.☹️
Your ex should contribute to your children’s upkeep so if he refuses to do so, contact the CMS – who can deduct money from his salary at source.
unfortunately, someone can not be made to see his children. I understand why you want to encourage their relationship but if he doesn’t want to , he can’t be forced.
I’d stop chasing him. Put in a CMS claim for maintenance and respond to him only if he is polite and respectful. Otherwise ignore him. Anyone who would do that to his own children is not worth the bother.
I hope it works out.