What should I do in this situation I really need help I don't know what to do?

My kids father let someone carried my kids on their income tax, and he does not take care of them. Matter of fact he has not saw them in 3 years they don't even know him. He let someone carried my kids because he can't because he owe money for child Support I know it him because he did it before and he has there ssn because it on our court papers and I guess he split the money with the person whocarry's the kids. I let my husband carry my kids I don't work I am a stay at home mom for now and so we can't file joint but now my husband is paying the money back I had no idea that my kids father has been doing this for years until the irs sent papers in the mail for 2007 , and the problem is that the IRS want tell me who carried them and when and why my husband has to pay money back. I know it was my kids father because he let someone carried them before without my consent. We sent documents off proving that we take care of our kids and everything I'm just scared that he might do other things to mess up there credit. So how can I protect my kids? What can I do legally, can I change there social security number ? Most of all how can I stop this for happening?
What should I do in this situation I really need help I don't know what to do?
You need to find competent legal help for all this, not a bunch of yahoos.
What should I do in this situation I really need help I don't know what to do?
Alert the IRS ASAP