I want to share iOS apps with my daughter
I have solved the issue (after a hour with Apple support, which was escalated to an Expert).

The short answer; I needed to log out of the Apple ID which I use for Contacts and Calendar, and log into the original "Primary" Apple ID.

I could not do this, because "it was already logged in". This was "resolved" by deleting the ID in Internet Accounts.

I could then login as the old Apple ID, enable sharing, logout, then login as my iCloud ID.

In the mean time I could not login because I did not know the answers to the "Security Questions", and was prompted to enter new answers (which seemed to be designed for an American teenager, not an ageing man who can't even remember his time at school). It then INSISTED I change my password, and refused to accept most of my compliant attempts, for some undisclosed reason - this is where Apple became involved.