Let them know you love and accept them no matter their weight.
Subtly help them get moving… do physical activities with them. Go for a walk around the block or at the mall. Let them think they are helping you stay healthy.
Help them find an interest or hobby. A child who has an activity they can "get lost in" is less likely to fill their down time with food.
Make sure they have clothes that help them feel attractive. Don't put off buying them clothes until they lose weight. The better they feel about themselves the more likely they will be to put food in its proper place.
Don't nag your child about his or her weight or criticize them about their appearance.
Don't compare them to other, slimmer children or siblings.
Don't be food or diet-focused yourself. If they hear you talking about weight all of the time, they'll be more inclined to make it their focus too. Make sure they know that the inner qualities of a good character and kindness are important and valued.
Don't make your child's food choices a focus with comments like, "Haven't you had enough already? Maybe you should think about skipping dessert tonight."
Make healthy and tasty food available for meals and snacks.
Don't make certain foods "off-limits." That will just make your child focus on that food more.