Why can't my kids make video calls, and how to allow specific contacts during downtime on iPad?

Hey peeps, I have a few questions. First is a kid messenger question. 2 of my 4 kids account CAN NOT make a video call. It goes to call then says call ended. My other 2 kids can video call no problem.

and my other question is, when my child’s iPad is on downtime, is there a way to have specific contacts that they can message while on downtime?

thanks,

[Re-Titled by Moderator]