Prepare a correct tax return that claims the dependents if you are allowed to. Sign and mail it. The IRS will investigate any duplicate claim. You may be asked to provide proof that you are the person who is qualified to claim them. You can also file an amended return for last year to claim them if you were prevented last year.
You can also request an IP PIN (identify protection PIN) from the IRS for a minor child. This is a 6 digit PIN that the IRS resets every year, and you can't claim the dependent without the current IP PIN (it will be mailed to the person or it can be retrieved on line). https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin
Lastly, if your child is the victim of identity theft, there are other steps you can take.
https://www.identitytheft.gov/
