Hey @noonoonoo! Welcome to the community.
No need to worry about your minutes, FaceTime will not use any on your plan. Data is another matter, however. FaceTime will use data on your plan unless your kid was using it while connected to Wi-Fi.
You can learn more about FaceTime here.
I hope this clarifies.
Happy New Year and Welcome to the community!
As noted in my reply in the other thread you posted, apparently, Apple might have changed the FaceTime interface which may inadvertently caused some calls to connect via the cellular networks rather than data or Wifi (see here and here). Is it possible that you might have connected those calls before the FaceTime feature fully loaded?
Unfortunately, the issue is happening on other providers as well. A simple Google search will reveal that it has happened with other providers (ie T-Mobile, AT&T, Koodo, etc).
In order to prevent unintended long-distance calls in the future, you might consider enabling Airplane or Flight mode on the device prior to making FaceTime calls. You could manually enable Wifi to connect the calls. Doing so should prevent the phone from connecting to the networks and therefore prevent unwanted long-distance calls.
Hope this helps 😀