Cannot use child's gmail address to add them to my family

I am trying to setup an iPad for my kid using an existing gmail account as their apple ID. I want to add them as a family member so that I can manage their device.

When I am adding them as a family member, I enter their current gmail address the click the link requesting a verification code, but that message is never received.

I have tried adding noreply and no_reply using both apple.com and email.apple.com domains to their contacts list on gmail. But still not working.