Currently you can setup alerts regarding usage that notify or disable a line for calls, texts or data.  Instead just an alert or disable, when a line hits a set threshold, it would be nice to have the option to limit.  Limit to other phones within your plan or even better yet, a whitelist of numbers.  A limit option would only be for calls and texts, but it would be a handy way to help manage the usage of children while still retaining the ability for them to contact you and vice versa.