My child is gaining too much weight. How do I help her without causing self-esteem problems?

Self Esteem. Try to exercise and eat healthy yourself, when we model health we are far more effective than lecturing our children. Go for walks together, and depending on her age engage in age appropriate physical activities and make sure you maintain a healthy life style as she is likely to follow your lead. Best.
Stop all sugar. The reason for obesity in the U.S. is standard American diet that spells sad. Get rid of all the "junk" particularly the sugar, soft drinks, (particularly "diet"} and commercial fruit juice. We raise our children with the idea that sugar is harmless. Please go to my blog "Oxygen the spark of life" that you can Google.