I think that obese children are doomed to be obese adults, because they learn bad habits from their parents. Obese children are unlikely to change their behaviors as adults, so most of the time, unless they go on the Biggest Loser, they are likely to repeat the poor choices imposed on them.
Childhood is a period of learning, after all. As a child learns and grows, they have plenty of opportunity to develop different habits. Given proper education, a child can learn that eating right and exercising is good for them. Besides, obesity is often a result of economics -- that is, cheap food isn't good for you. As children grow into adults, it's possible that their economic situation will change, therefore affording them the opportunity to make better nutrition choices.
Just because you are obese as a child does not mean that you will be as an adult. I know many kids that were obese but once they hit puberty they grew taller and became thinner. Some people are just heavy as children but grow up to be more slender.
I do not think that obese children are doomed to be obese adults. I think that while an obese child's chances of being skinny in adulthood is slim, there is still a chance that with a good diet and exercise regime, they can turn things around and get skinny when they grow up.