I would personally recommend taking a look at Family Allowances from T-Mobile. For a small cost you can control their minutes, phones they call, data and more for these lines.
You can even put a schedule on what times they can use their phones.
Hope this helps
I used the Family Allowances last night and today to block certain time in a day and I blocks the phone from receiving calls but not the use of data. I then click on the disable phone tab and there was no change, phone still able to log in on the internet. T-Mobile tried to help and so far no change has happened.
There are a ton of apps out there that come from outside T-mobile that can you give you this control with varying levels of cost. I will link a website that shows you some of the best as well as the cost and such.Best Parental Control Apps 2018 - iPhone & Android Phone Monitoring However, I personally believe it is not worth the time, cost, and effort to go through with this kind of stuff as there is always work arounds, should the kid be tech savvy. I am not sure the age of the child in question but I feel going old school is the easiest way to handle things or just make them start paying for there own line if they are old enough to work.