How do I give my child my old phone with her SIM card?

I have put her SIM card in my old phone, but the phone still retains all of my stuff and won't call out or text. 

How do I make this switch? I'm assuming that I need to factory restart the phone, and I was going to do that today. But, I thought I'd ask first so I don't flub something up...…

The old phone is a OnePlus 7T