What bothers me more about the episode is the way that after hundreds of years, the children are still children. You'd think after a while they'd be adults in childrens' bodies. Unless you want to make the brain development argument.
Maybe society has a lot to do with it. With no examples of adults around they might have arrested development. Still I think they would be different from normal children. I can't really see a four year old still acting the same after 300 years, no matter what the environment. Maybe the disease also stopped their mental development?
In regards to the food, with say at least 90% of the population wiped out - all adults, teenagers and the very young eventually I just can't see them surviving on canned food. Unless the town they lived in was a distribution centre for canned goods and had a big warehouse full of them.
Maybe the only kids who didn't starve were in farming communities where the kids knew how to plant each year.
I can see how these 20 or so kids might be the only ones left. Disease, fighting, accidents, starvation, shock may have killed off everyone else in 300 years.