So what would be a fair tip for my family to pay
10 yr old boy
8 yr old girl
6 yr old girl
3 yr old boy
I'd imagine that at least two of the kids will be in their own cabin (unless you've booked a family suite). So, in my view, unless service is awful. You should pay the full $10/pp/per day for the first four... $5 pp per day would likely be acceptable for the remaining 3.
Though with a 6 and 3 yr old they could require more work from the staff than if there were adults.
Personally I think I'd leave the $10/per person/per day in place for everyone but the 8 month old. That's $420 in gratuities total for a family of 7. Not outrageous at all in my view.