I actually don't disagree that the children didn't need to know about the ins and outs of your financial battles.
However, it's not narcissistic to tell the truth, even if it might have been a bit unnecessary. It's also extremely unlikely that your children went from having a close and loving relationship with you to moving in with their Dad purely because he told them about this.
The chances are that this is the last straw and they are sick and tired of your behaviour and your attitude towards their other parent. Stop calling their Dad a narcissist (even if he is, although all of his behaviour that you've described in this thread indicates a good Dad and ex partner who has done right by you and your kids, but refuses to be taken advantage of) and for the love of god, PLEASE stop thinking you deserve money from him as some sort of compensation for parenting your own kids IN THE PAST.
NO-ONE agrees with you. Not your kids, not your ex, not Mumsnet. Doesn't that tell you that you must have got it wrong and your thinking is distorted?
Your kids are not coming back whilst you still think you are right and their Dad is in the wrong. Even if he was wrong to tell them, you were more wrong to do it in the first place and whilst you are focusing on what he's done wrong you're not taking accountability for this situation, which is not only entirely of your own making but was accurately predicted and described to you before it happened. We were right then, and we're right now, when will you start listening?