how do i remove face id on child account

You’ll have to do it on the child’s device. To do so, you must first go to Settings > Screen Time > Content and Privacy Restrictions > Passcode and Face ID > Make sure changes are set to Allow. Once that is done, you can go back to the Face ID and Passcode section within Settings and Reset from there. Once finished, return and restrict again.

As already stated FaceID exists ONLY on an individual device.

To disable, simply reset the device’s FaceID (which erases all stored “facial data”) and don’t setup.

Restrict the child from resetting thru the parental controls.

Use parental controls on your child's iPhone or iPad - Apple Support

Face ID is tied to the physical hardware (the iPhone/iPad Secure Enclave). It is not tied to the account. Can you give details on what device you are attempting to remove Face ID from?