Sometimes kids learn from their parents and say that they don't want to be like that. So for example, if the mother and father can't go on elevators at the same time since they exceed the maximum weight occupancy limit, then a smart kid would make different decision and change their lifestyle.
Even if they see their Dad walk up the stairs and his nose starts to glissen a bit, he might think, hey, I don't want to be fat and get all sweaty just walking up stairs. Then they change their ways.
The fat parents still died to soon. Sticking around would have helped the children in this case.