You can report this to the IRS: https://www.irs.gov/individuals/how-do-you-report-suspected-tax-fraud-activity.
Unfortunately, if your return is being rejected because someone claimed your child fraudulently, you cannot e-file your tax return this year. You should mail your Tax Return to the IRS.
Once the IRS receives your return, they will send out paperwork to you and to the other person they are showing claimed your child in order to determine who should be claiming them. Once you prove you are the rightful claimer, they will issue the refund to you and rescind it from the other person.