Is this blackmail?.....

I wish for my ex-wife to allow me to claim one our children on my taxes, but she refuses. She tells me that she is going to claim both of our kids and let her mom claim her eldest child (who is with another father) Her mother has no legal right to claim her granddaughter, my ex is letting her do it so she can receive the extra tax rebate/credit. Would it be considered Blackmail if I were to tell my ex that if she allows her mom to do this, they are both commiting a federal offense, but I will not report it if she were to let me claim one of our children? I dont want to do anything illegal...