My ex is threatening to claim my granddaughter on his return due to debt he owes this irs. she lives with me?

My granddaughter is 6yrs old and I have legal guardianship of her. She has been with me for 8 months and I provide her financial support. My ex is in debt with the irs and is saying he is going too claim her to help his debt. what recourse do i have if he does this?