How can I limit the usage of my kids’ cell phones to certain hours?
Verizon and T-Mobile both offer this service, and I would be somewhat surprised if AT&T and Sprint don't offer it. It's a paid service. You can restrict when your kids can use their phone and who they can call or receive calls from, with some exceptions - check with your wireless carrier for more info. It's $5 a month with the two I checked with.