There are phones that have a specific Kid Mode on them to prevent a variety of things. But for what you want to do, it might be best to simply call into Tmobile Support and ask them to block those things from whichever line you want.
I think the safe thing to do is limit the content, don't enable WiFi . heck don't include a data plan period lol. But blocking texts etc is something you should ask Support to do when you call in. I think it can be done . I know they can block a variety of things.
also, you might look into some paid apps in the Play Store that have some really good features as far as parental controls are concerned.
My sister has a family plan with AT&T and they allow the blocking of MMS, but allowing SMS for her kids (it was funny when her youngest wanted a ringtone and she told me to text it to them, then realized it would fail). If you disable data on the phone, MMS will not work (there's many threads about people not being able to MMS
Data should be blockable per line with T-Mobile.
Apps are the hardest. If it's installed on the phone, T-Mobile cannot designate whether or not it will run (if it requires data, that's one thing, but preventing your kids from playing Angry Birds at school is impossible if it's installed)
Deacon is right with the fact there's many apps out there to help with parental features. You just gotta make sure your kids aren't savvy enough to figure out how to uninstall them (which, I guess, means persistent checking as a parent to make sure everything is as it should be -- my sister does random audits of her kids's devices -- and they lost their privileges when they installed a third party texting app )
I pay $6 a month for mymobilewatchdog ... I can see every contact added, every call and text sent and received, every picture sent and received, time block, and location of device ... I absolutely love this app !!!
Sounds pretty cool!
Does it tell you when your kids disable it and re-enable it?
Or when they create a secret Facebook account and link it to their phone?
(Sorry, my sister actually dealt with the last one.. and there was a lot of furiousness when she found out her daughter was copying off her boyfriend in school and dropping F-bombs left and right)
The short is nothing is going to be 100% foolproof. The best parents can do these days is to be diligent with their children, tell them they're not allowed to delete texts until you've compared them to the usage on the bill, and do random searches of their devices to make sure they're not doing anything inappropriate.
And, make sure your kids understand that you were once a kid yourself, so you know everything they would try. And that technology doesn't let you hide all your steps!