Golden rule - don’t share your AppleID.
Every user should use their own unique AppleID. Sharing an AppleID risks significant information leakage between devices and will introduce major unexpected issues - in particular with Apple Messages and Keychain.
As for your Keychain, if your AppleID has been shared with another device that you intend to be used by somebody else, unless you follow the correct procedure, your keychain and other personal information (such as you Calendars and Contacts may be unexpectedly merged with another AppleID account. If the merge occurs, the only resolution is to manually remove every merged entry from whichever dataset the merge occurs.
A merge will occur if you sign-out of an AppleID on a device and elect to keep a local copy of personal data - and subsequently log-in to a different AppleID in the same device. At the point of initial log-in, any locally stored data is merged with iCloud. Beware - if this occurs, you have to manually find and remove every item that you didn’t want to share!
Your best route forward is to prepare the the iPad to be used by your Son by following this procedure:
https://support.apple.com/en-gb/HT201351
Next, ensure that an AppleID is created for your Son. If your Son is a minor - as you have already set-up family sharing - you should create an AppleID from within family sharing. If you Son is older, then create an AppleID account using the standard process - and add this AppleID to the family group. Family Sharing will permit selective sharing of Apps, storage, media and family Calendars etc.
More info about family sharing and its setup can be found here:
https://support.apple.com/en-gb/HT201088
https://support.apple.com/en-au/HT201085
Using your Son’s AppleID, set-up the freshly prepared iPad. If you have created a new AppleID, your own personal data (including keychain) will not be visible to your Son.
I hope this information and guidance proves to be helpful in resolving any issues that you might have.
