I can recommend a phenomenal family lawyer with real experience and expertise in contending with abusive (forgive me, I know that's a hard word to hear in this situation) men. She's also excellent on the finance side. She has acted for a friend with a horrendous ex, who was financially and emotionally abusive and also saw her as a walking cheque-book. Her solicitor was very supportive - and not out to increase billing by increasing the conflict, either.
Emily Watson at Raydens
. She's Resolution registered (they are solicitors who act in family matters who are committed to avoiding conflict wherever possible - agree with those above that it's toxic in family disputes, especially), a qualified arbitrator for children's disputes and she's highly ranked in all the legal directories. She has real skill and experience with very high conflict situations pertaining to children, especially, and will absolutely understand your anxiety around their safety.
Honestly, my friend said she was a bit terrifying (though very kind) but it sounds like your ex needs you to have a lawyer like that.
I am so, so sorry you are in this situation.