Does being sedentary make kids fatter? Not much. In an 11-year cohort study, it seems instead that being fat makes kids sedentary. In an effort to find out why attempts to reduce obesity by increasing activity levels averaged having kids lose an ounce a year, they discovered that "Physical activity had no impact on weight change, but weight clearly led to less activity."
The authors attribute obesity partly to "portion size, calorie-rich snacks and sugary drinks," but this just pushes the question back a step. Why do some people eat different foods and bigger portions? The authors say "early feeding errors seem crucial."
I hope this doesn't mean I'm go blame if my kid is fat, because I fed it wrong as a baby. I have enough guilt already. The actual article is behind a pay wall, so all I have is the EurekAlert linked to above, so I guess I'll speculate what it means.
It could mean that what a kid is fed early in life turns some genes on or off. It is not implausible that the amount of fat and calories and acid, and god knows how detailed it might get, might influence the expression of genes that make you crave fat, or salt, or custard-filled chocolate éclairs more than other people. It could be epigenetics in action, to go along with the genetics we know are also involved.