It could be most anything. Not that that helps you. Okay, let me think...
The kid may have added something to your phone. Perhaps fiddled with the apps and they're all on now, in which case you get go in and shut off most of them. Review your list of apps. Check if any have been altered to where they can access your files, address book,etc. etc.
How reliable is this cellphone repair shop? Are they new or a pop-up?
National or at least locally known name?
Then again, it could be the kid who is the problem. Either way, it's a problem.
I have a friend who has an i6 and it's had several difficulties. lately it starts dialing the keypad while he's in a call. It also seems to drain rather quickly. He ran down the battery from 100 to 23 in about an hour and a half the other day. (He was on a short trip out of town)
Strange coincidence, he went to the Apple store yesterday. Time will tell as to whether it has been fixed or not. (get more than a bit irritated by calls suddenly ending in a short time and him having to recharge to even call back)
I don't know anything about i-phones in general as I've gone the Android route. Wishing you the best on this.