Unlock my iPhone 11

1.

(< iOS 15.2) See this Apple article: If you forgot your iPhone passcode

If you forgot your iPhone passcode - Apple Support

(iOS 15.2 and newer) 

How to remove your iPhone, iPad, or iPod touch passcode without a computer

How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support

2.

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 or iPad - Apple Support

Then you have to remove the iPhone 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

Then your grandkid can sign in with their own Apple ID.

If it is your phone then you should know the password for the Apple ID that it is activation locked to.

Settings, General, Transfer or reset iPhone, Erase all content and settings will wipe the phone ready for them to set up using their Apple ID.

Apply the password, then the phone is unlocked from activation lock.

If it is also locked to a cellular provider than only that provider can unlock it. Contact them or use the same carrier network.

If they try to set it up and create a new Apple ID at the same time, they may get a message when setting up that it has created too many Apple IDs. If so then they will have to create the new Apple ID in https://appleid.apple.com and use that