Family Sharing Members and FaceTime

I have been searching for hours, but I cannot find the answer to my question anywhere online.

I have two children, under 13, set up with Apple IDs in Family Sharing. I want to allow them to use my FaceTime on my iPad and my MBP, but it won't let me sign them into FaceTime.

They are required to have their own Apple ID for their devices that the school provides. They have used those Apple IDs to communicate with friends. I want to allow them to use FaceTime and iMessage on my iPad without requiring them to give all of their friends my Apple ID. However, I do not want to sign out of my iCloud account on my iPad.

Here are the errors I am receiving:

1) When I use my son's Apple ID (with me signed in to iCloud), it says, "FaceTime Activation This email address cannot be used for FaceTime."

2) When I use another email address that my son has (it is verified through HIS Apple ID) it stays stuck in "Verifying." If I click the option to resend the verification email, I never receive an email.

I do not want to have everything tied to my own Apple ID. I do not see why Family Members' Apple IDs can't be used on my iPad with my Apple ID signed into iCloud. Am I doing something wrong?