How can I encourage my kids to eat healthier without giving them complexes about their weight for the rest of their lives?

I have 2 kids, a 7 year old girl and a 12 year old boy. My son is overweight and my daughter is obese. Not like Honey Boo Boo big, but still obese according to her BMI. She has always been big, at the top of growth charts for height and weight, but now the height part is slowing down and the weight part isn’t.I am trying to make changes to their diets to help them lose weight, or at least not gain weight while they’re growing taller, as well as be healthier in general. They will both snack on junk food all day long and I’m struggling to help them understand that this is not a healthy behavior. I give them healthier alternatives, which they will eat, but then they want to reward themselves by eating something unhealthy. My daughter will also sneak food, so if I have any candy or that sort of thing in the house she’ll eat it secretly.I need advice on how to discuss the short and long-term effects of their diets on their health in a way that won’t totally mess them up forever. I was chubby as a kid, and my parents’ efforts to fix the problem did more harm than good. I don’t want my kids to feel like I did, not do I want them to end up struggling with disordered eating for the rest of their lives.I really want to teach them good eating habits that will follow them even when they’re not in my house. Any suggestions on how to do this are very appreciated!Edit: my kids are only with me 50% of the time. They’re with their dad the other 50%, and he’s not willing at this time to make changes to be healthier. For that reason, I would really appreciate ideas on how to get my kids to WANT to eat healthier food so they’ll do it when they’re with him as well, rather than just eating healthy when they’re with me because they don’t have a choice and then eating sugar nonstop when they’re with him because they can. via /r/Parenting http://bit.ly/2WDVNaM