Vlad This will vary by family I'm sure, but for my household, I'm by far the heaviest search user. I suspect that the people who are currently signed up for Kagi are power users who care deeply about search, where as our families are probably not on average.
Obviously would have to run the numbers on it, but I suspect you could get away with a pretty good discount for the other users in the average household.
As a separate item, I think a family account should have a way of enforcing safe search for specific sub-accounts.