Port Book Child Order to Drupal 8+

Drupal 9 + 10 support.

Fixed an incompatibility I ran after upgrading my WIP site to D10.2; I don't know which core version changed this, but it works now.

Minor fix for a bug that was showing with D10.

Minor comment fix.

joachim: I'd be happy to be a comaintainer so I can commit this and tag a release.