There were never more than 2 students per instructor at any time. The the 2 older kids did 18M with an instructor and no other students (just a lurking videographer and the kids dad who is an experienced old NAUI guy...not sure if they would be any help if a problem occurred). On the under-12's first open water dives were 1:1 and never stepped off the boat at areas deeper than 12M.
I'm probably not qualified to assess the instructors but the kids seemed very comfortable, confident and mature on their dives. The instructors commented that their previous confined dive skills were under-reported and they were doing better in their open water dives than many adults at the same stage (and seconded by other instructors who we weren't paying ;-)).
I actually think they benefitted by doing their first confined dives at the chaotic Blue Zoo in Beijing where they had to deal with distracting sharks (my avatar), moray eels, and bikini-clad Chinese mermaids, all while watching tourists snapping photos of them.