1.
Have you forgotten your Apple ID?, Check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID
If you forgot your Apple ID email address or phone number - Apple Support
OR see -> Having trouble signing in?
https://iforgot.apple.com/password/verify/appleid
If you've forgotten your Apple ID email address or phone number
If you've forgotten your Apple ID email address or phone number – Apple Support (UK)
Have you forgotten your password? -> See this Apple article: If you forgot your Apple ID password
If you forgot your Apple ID password - Apple Support
Then see this Apple article: What to do after you change your Apple ID or password
What to do after you change your Apple ID or password - Apple Support
2.
Then read -> this Apple article: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
Then you have to remove the iPad from your account, see this Apple article:
Check your Apple ID device list to see where you're signed in
Check your Apple ID device list to see where you're signed in - Apple Support
After that your grandchild can sign in with his Apple ID.