well families come in all varieties. Even in summer the plane is not full of parents and two minor children. I don't think triplerotor was really complaining about a 15 year old son or daughter being separated but was talking about very young and necessarily dependant children. It may be that some parents are happy to delegate the safety and security of their children to another responsible adult who is themselves happy to accept the responsibility however that should not be assumed. It is interesting to note that many airlines will not allow a young child to travel unaccompanied even if another adult is prepared to assume a temporary guardian role ( and perhaps rightly so), but seem to have fewer qualms about separating a child onboard !
As already stated this is a problem that does arise but need not cause insurmountable difficulties. I do believe that a high degree of effort goes into ensuring parents and young children are seated together and the problem only arises in a few cases. However when it does arise it should be resolved.