On a serious note, I do think some children’s weight can be down to the genes they have, for example my son eats alot of food, in fact he is always saying he is hungry and the amount he eats you would expect him to be the size of a house but he is very slim.
Maybe this has something to do with genes as his father is just the same, and I can imagine if this is true then maybe the opposite can occur and a child be obese because of his/her genes.