Your account ID is individual to your account. It’s mostly useful for our customer support team when troubleshooting.
So if they ask for it can you open Screen Time on the iPad/iPod/iPhone press the More button at the bottom and you should see the Account ID on that screen.
Summary

Article Name
Where can I find my account ID?
Description
- Open the app on your child's Apple device
- Tap on the More button at the bottom