How to enable children to sign out their Apple ID

Hello yingmei257,

Welcome to Apple Support Communities!

We want to help so you can sign out of the Apple ID on the iPhone. See these steps for how to sign out:

"Sign out on your iPhone, iPad, or iPod touch

1. Go to Settings, then tap your name. 
2. Scroll down to the bottom.
3. Tap Sign Out.
4. Enter your Apple ID password and tap Turn Off. Forgot your Apple ID password? 
5. Turn on the data that you want to keep a copy of on your device.
6. Tap Sign Out.
7. Tap Sign Out again to confirm that you want to sign out of iCloud."

This Apple Support page has the full details: Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support

We hope this helps to sign out of the Apple ID on the device like you want.

Please let us know if you have further questions or see any errors when trying to sign out.

Cheers!