If you want to know how to keep your child from becoming obese, the best thing you can do is to lead by example. Teach them from an early age the importance of eating right and getting plenty of daily physical activity.
We are creatures that can be taught many things. Many of the things we like or dislike are actually just habits. If you teach your kids to like fruits, vegetables and to lead a healthy active lifestyle, they will. Of course, they will make their own decisions as they get older but that early training will set the stage for life long good habits.
One of the worst things you can do is to be a hypocrite. This is true not only with diet and exercise issues but with your overall lifestyle too. For example, if you are a smoker, it is OK to tell your kids they shouldn't smoke, but if you don't want to seem like a hypocrite you shouldn't just 'order' them not to smoke (which doesn't work anyway) it's much better to talk to them honestly about the dangers of cigarettes and how you wish you had never started, etc.
lead by example, always. It's also helpful to workout with your kids. From a young age include them in the activities that lead to a healthy lifestyle. Go to the gym together, take a workout or fitness class together, take martial arts or boxing classes together, go for a swim or play basketball together. Just get in the habit of spending time getting some exercise together.
You can also include them in the food planning. When they are old enough go through cook books with them and ask them to help you find new recipes to try. Let them come with you to the grocery store and teach them how to buy fruits and vegetables.
Go to farmers markets or health food and organic stores with your kids. The earlier you start these habits with your kids the more ingrained it will become (it's also a wonderful way for you all to spend time together).
Many of these ideas can help you set your kids on a path that will be the foundation for a life time of better lifestyle choices... and it can be fun too.
Always encourage your kids to try new things, to try things that are healthy and good for them. Sometimes parents inadvertently blow off their kids and their ideas without really meaning too. For example, if your kid is kind of a klutz and they come to you and tell you they want to take martial arts lesson, don't laugh and remind them that they are a klutz, instead encourage them to give it a try.
You may think that you are protecting them from being hurt emotional, but you can't go through life that way. You won't always be there and they may just be good at it. Even if they're not, you can still help them find something that will work for them.
It's easy to learn how to keep your child from becoming obese, the best way to do that is to be proactive and teach them good habits from the start.