How do I report someone illegally claiming my children on their income tax return to the IRS?

I know that my husband's mother is claiming my child, she had asked me along time ago but I told her no and she still claims him I did not give her permission nor i did not gave her my son's social, I told my husband I don't want her to be using my child & then she doesn't give me nothing she doesn't even support my child never has helped me out and what can I do?