What do you think the single biggest thing is that can be done to prevent childhood obesity?

I just read an article that childhood obesity is expected to rise drastically by the year 2030 across the world. In the past 30 years in Canada rates of childhood obesity have tripled and the issue seems to be getting worse rather than improving. Children who are obese are more likely to suffer from a variety of health problems for the rest of their lives. Childhood obesity is linked to a variety of health problems like type 2 diabetes, high blood pressure, breathing problems, bone and joint problems, depression, etc. In the article I read it stated that obesity is now becoming a bigger concern in developing countries as well and often developing countries have less access to doctors and medical help so this is an even bigger issue in these countries. What (if anything) do you think could be done worldwide to reduce the rates of childhood obesity? What are the reasons that the rates of childhood obesity are increasing and what can be done to help?