I was worried for a while. We have a large family (4 kids) with LOTS of Apple devices - so far 4 iPhones, 3 iPads, 5 iPod Touch, 6 Macs - and I really don't think it is reasonable to pay twice for e.g. games and books that we all share in the household.
As long as you can sync to the same physical computer, I think that is fine. I can see they have to limit the iCloud syncing and 10 is as good a number as anything IMO.
Time to teach your kids about buying their own stuff. Also, your definition of reasonable is irrational. How many duplicate apps and books could you afford if next time someone wants a new $1200 Mac, you just skipped it for a year?