At home the router could be a better place to "filter" contents for kids, especially they may use both cellphone and PC at home. When they're out on 4G network

At home the router could be a better place to "filter" contents for kids, especially they may use both cellphone and PC at home. When they're out on 4G network, then you do need those apps to help, unless the 4G operator has a safe-filtering-for-kid-phone-numbers