Q: How can I add more than 6 members to family sharing

If six is the limit, then you cannot add another. Apple sets those limits.

Family Sharing

Family Sharing and Apple IDs for kids

A Guide to Family Sharing - iCloud - Apple Developer

Sharing purchased content with Family Sharing

I think the only current work around is to graduate one of the older members of the family to their own account.  You lose the family sharing privileges of apps and media sharing but would be able to protect the youngest in the family.  Also, having each parent have their own Family sharing can enable sharing for 10 kids, but you have to buy everything twice. Not ideal but still probably the best game around for sharing.