If your child is a victim if Identity Theft, you can report that to the IRS and they will mail you a special PIN that is required to file your tax return. Someone that does not have that PIN can not claim your child as a dependent on an e-filed return.
https://www.irs.gov/newsroom/taxpayer-guide-to-identity-theft
There is also a new voluntary opt-in PIN that does the same thing starting for your 2020 tax returns. The IRS won't have it ready until mid-January, so whether or not it will work out well for this year or not is questionable.
https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin
