Nick Riviera, it isn't always possible to please the last pax to check-in regardless of how many offspring they have. I remember getting a real ear bashing from a normally placid No1 when she returned from her flight. She was calling our handling agent all manner of names re split families, children in emergency exit seats etc.
I asked if she was willing to wait whilst I got all the details from the H/agent and went to find out what had happened. It turned out that on this flight there were very few non family groups and as the check-in sequence numbers showed they had done all possible to keep only able bodied pax in the exit rows but apart from about 6 pax they were all family groups. Then the seats together ran out about 2/3rds of the way through c-in, and there weren't any non family groups who could be re seated to overcome the problem.
In short everything possible had been done, but the pax who showed up last had no option but to be split. With juggling they managed to have 1 adult + 1 child but not families together.
It is rare for an airline or handling agent to deliberately give pax reason to write in and complain!