It's easy enough to prevent children from phoning others or hirering staff: just don't direct them to do so. On their own they may occasionally play on their phone or at best text a little.
Greying out the option would only serve to take away player freedom. Restricting young kids irl is a whole different issue from restricting player freedom. Not everyone is always playing families the way they'd raise their real children. This is also why the deleting of neglected children is so awful. It punishes the player for something he probably INTENDED to happen for the sake of their story.