Data usage-need to control

You have several options. One is to have them pay off their phones and boot them off your shared plan onto their own prepaid plans. You can also teach them how to monitor how much data they are using, and let them know that they have a limited amount and once it's used up you will turn off data for the rest of the I'll cycle. 

Then do it.

You can turn off data to any line on your account by just logging into your account, managing their particular line. One of the options is to suspend, but you also have the option just to turn off data.

It works for mine, the first month they were without data part way through the month they learned how to keep track and stay under their allowance.

Another option is to take a look at the cost of plans. If you're on an extremely restricted data plan, it might be time to consider moving up. These great big screens take a lot of data just to function. Most of the current unlimited plans are cheaper than old plans.  The option to mix data plans means that you could put each line on a different plan.