The Stop Obesity Alliance has a guide for parents wanting to help their children who've developed or may be developing an eating problem. I think it is worthwhile, and recommend it to you, both for reading and sharing.
Most consistently stated point: Approach obesity as an issue of health, rather than of appearance. It acknowledges bullying, which was definitely a facet of my obesity experience growing up, and of course, nobody bullies someone for their lack of healthfulness.
I agree with the emphasis on framing it as a health issue, but the bullying angle helps illustrate why no one thread of advice is enough. When I was young and kids made fun of me, my parents told me to ignore it. That might have worked for other kids, but it didn't work for me.
Honestly, I don't know if I could have been helped during that time. My folks were certainly aware of my weight problem, didn't model one, and tried several tacks to resolve a problem that only got worse until I reached my 30s.
As the parent of a 3-year-old, I'm convinced that starting early, before lusts can develop for refined sugar, flour, and processed foods in general, is crucial, but I also acknowledge that I don't know what I don't know, and have no illusion that I'm the one who's going to do it all right.