Need Whitelist-Only Email Gateway in Cloud for Children Email

[Widmark]

I want to introduce my kids to email but they are young... So want to limit inbound mails to only those from relatives and friends email addresses. This is much simpler than a typical spam gateway since ALL mail would be blocked except whitelisted email addresses. Does anyone know a way to do this with a free or inexpensive cloud gateway product? I am using Zoho Mail which allows third party gateways... I just can't find one that fits this description.

Thanks.

P.S. I am using Zoho mail since I can use it with my own domain. I do not want to use a different mail service.