Some employees may feel that they need their phones, in case there are emergencies with their kids, for example. Being unduly harsh in your policy will likely only lead employees to not follow it and to general unhappiness.
However, regardless of how reasonable your policy is, it’s likely that keeping personal device use to a reasonable level that does not interfere with work, will be a struggle for some employees.
That being said, if the job involves driving or working with machinery, it’s reasonable to require phones to be kept in a locker and only accessed on breaks, and any infringement of this policy should be treated just like any other and subject to progressive discipline.