Most people don't start out fat, they let themselves get that way and they can change if they want
Children don't let themselves get any way. Their parents let them get that way. Once -that way- it becomes so much more difficult to change, even as adults.
As for fat vs. obese, many children who are not actually fat, but healthy with 'baby fat' are considered fat because of what society dictates is attractive. And once self-esteem is gone, destructive behavior will begin or worsen if it was already there to begin with.
Tammy