The only way to resolve this is by mailing in the tax return.
When you mail in the return, what will happen is the IRS will verify that the child was living with you, and ask for proof. The IRS will review all information and will award the claim accordingly. Of course, this process does take time, but it is the legal remedy the IRS provides.
See this similar response from TurboTaxDanielIV: https://ttlc.intuit.com/replies/6550169
