There are other means for a child to make contact aside from personal mobile devices. Conventional phones, for example. At our local school, all mobile devices are put in lockers and are not allowed in class.
And in an emergency you can walk to the locker, retrieve the phone, and make a call. It makes no sense to require landline use in an emergency when mobile phones provide a valuable backup.
What makes no sense is y'all allowing your children to be spied on by Google/Apple/Microsoft/Facebook/Snapchat/NSA/GCHQ 24/7 on an age when they don't know better.