He was paying maintenance. You went for more, and you did lie to CMS. You posted here about how you shared care etc but you told CMS that you did it all.
We all told you exactly what would happen. You didn’t believe us. When he went for child benefit, we told you exactly what would happen but you wouldn’t listen. Everything happened exactly the way every poster said it would. But you didn’t listen, and you still don’t listen.
You went after money you weren’t entitled to, by lying or omitting the truth. You have never once taken responsibility for that or apologised to him.
He is now entitled to maintenance from you. He isn’t going after anything. He is entitled to it. You’re the one who went after money you didn’t deserve and weren’t entitled to.
It sounds like he told the kids the truth. And they’ve made a decision based on your awful behaviour.
Stop lying, to yourself and to everyone else. The only way you might get your kids back is to fully admit how wrong you were right at the start, and to fully admit how wrong you were when you fought the child benefit claim and for not paying the maintenance your ex is now due from you.
If you want your kids back then you need to take full responsibility for your behaviour. This isn’t CMS fault; you lied to them too.