Q: How do I restrict access to my child's iphone use?

It depends on what you want to restrict. You can go to Settings/General/Restrictions and set many restrictions. If what you want to restrict is not there you see what Sprint supports. AT&T and Verizon have services that can manage a number of restrictions on use. There are no apps that can restrict iPhone use, as apps are not permitted to meddle with iOS or with each other.

It all depends on what you have access to. If you know her Apple ID password, you can go to iCloud.com and erase the device. Then set up with your Apple ID. Or if you know her passcode, you can set one of your own choice.

Again, "restrict" can mean many things.  If you choose to only let your child use one app, this will do that.

Use Guided Access with iPhone, iPad, and iPod touch - Apple Support