I'm not sure what you mean by "switch to her Apple ID"? On the watch? On your iPhone? What does being 8 years old have to do with what you want to achieve? Why do you need her settings?
As far as the watch goes, all you can do is unpair and reset the watch and then setup the watch with your Apple ID. See:
What to do before you sell, give away, or trade in your Apple Watch, or buy one from someone else - Apple Support
Maybe you should see:
Set up Apple Watch for a family member - Apple Support
With Family Setup, your family member who doesn't have their own iPhone can use their Apple Watch to do things like make phone calls, send messages, and share their location with you.
After you set up a watch for a family member, you can use your iPhone to manage some of the watch's capabilities. And see the 'What you need' section in the above link.