My teenage daughter won't let me into her phone, can you?

Depending on the situation and age of the child, you might consider implementing controls to prevent your child from changing their AppleID password and device passcode.

Otherwise THEY can restrict YOUR access.

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

Only if you actually take physical possession of the phone.

Not without her passcode.

As KiltedTim is saying, you need her passcode to get into the phone, assuming she uses a passcode, and then you can look at what she is doing once you are in.