My ex-wife she sold my children's s.s.#s to someone , what can I do?

Contact the IRS as this is illegal and she can be prosecuted.

Agreed.

However, you may want to give her the opportunity to fix the problem before you get the IRS involved.

my ex-wife she sold my childrens s.s.#s to someone , what can i do?

What do you mean by sold SSNs to someone else? That someone may qualify to claim your children as dependents if they lived as member of someone's household for full year.

Only if you qualify to claim your children as dependents, then you may also claim them. IRS will figure out who qualifies. One of you will have to amend the tax return and pay interest and penalty.