if it is the IRS (unlikely to be your state) asking for proof then things like pictures of their room with you and your husband in the picture with them, medical bills (even 2020 and 2022) you paid for them or a letter from their doctor, signed affidavits of neighbors that the kids live with you, birth certificates, etc. the notice should contain these suggetions so if it does not be wary.
you may want to file an identity theft affidavit for each of your kids. the scenario I'm thinking of is that someone got hold of their IDs (SSN) and filed a return claiming them. you also filed claiming them. thus the IRS has two returns claiming the same kids.
could both you and your spouse have filed tax returns for the same year?
