You can file your tax return including your children as dependents.
If you try to efile your return and it rejects because the children's social security numbers have been used on another tax return, you can mail in your return with a statement that the return rejected when efile was attempted and that you have the sole authority to claim them on your tax return.
Attach any birth records or other documentation supporting your claim. T
The IRS will sort it out.
You will eventually get what you are entitled to and the inlaws will have to repay what they are not entitled to have.