What happens if someone files taxes on your child just so you can get more money my ex would always talk her mom into this I told her to stop but still doing it

What is the court ordered custody arrangement? The parent who has the child more than half the year gets to file. If that is you, then you file. Eventually the IRS notices and they find out who has the right, and the other one gets charged for all the back taxes.

We have 2 kids I have the oldest and I know my ex can file but she's letting her mom claim my other daughter she doesn't live with them

So who else is claiming her and WHO is she living with??
You have 2 choices: wait for the IRS to find out or tell them yourself. But you need to do it more clearly than you are here, LOL.