It is a concern and they do adjust for this. For example, undersold flights or flights with many absent passengers will temporarily redistribute passengers dur

It is a concern and they do adjust for this. For example, undersold flights or flights with many absent passengers will temporarily redistribute passengers during take off to ensure "balance". On a recent flight from Minneapolis to Helsinki, we were extremely underbooked but the distribution of passengers was fairly even - the announcement from the flight crew was that we could move seats but only once we were at the cruisin altitude. I assume that there is some balance struck with averages on children, and it would explain random seat reassignments at the gate.