How can I prevent kids from adding Contacts on devices?

I work in a children care home and we use screen time - including only allowing us to add in numbers that approved and safe.

i have been using this for a while now and thought I’d figured it out without the kids being able to over ride me! But it turns out that when children go into “lists” within contacts and select Gmail they can add in who they want and which ever number they want! I can’t seem to find an answer and hoping someone may know how I can get rid of this loop hole! Would appreciate and advice. Thanks in advance!

[Re-Titled by Moderator]