I recently read an article about the situation here in Britain and how the number of seriously obese under 1 year olds has increased in recent years! Unbelievable. In addition to the other reasons given above such as the inactivity of children, maybe there is an element of education that we need to consider as well. Let us remember that with the arrival of convenience and fast foods, knowledge such as cooking from scratch and cooking healthily is something that is in short supply.
All the above posts are helpful. I particularly agree with the idea that kids tend to be less active today than they were decades ago. Most of the playing I did as a boy involved moving around a lot outdoors; much of the playing that takes place today seems to involve playing video games.
I agree with all of the above posts and would also give some attention to the increase in marketing done TO children for foods that are not healthy. Fast food chains have received criticism for the use of toy prizes in their meals; there is more children's television programing on 24 hours a day, and the advertising of high sugar cereals and things like chicken nuggets and pizza rolls are constantly baraging children who are additionally probably watching too much television.
There's plenty of room to share the blame when it comes to obesity in children. Parents are primarily at fault, since they control what foods are bought and prepared in the house. Additionally, parents should closely monitor their children's food intake, whether it is in the home or elsewhere. Parents are also responsible for overseeing their children's physical activities, and too many children spend their free hours in front of the TV or computer instead of getting exercise outdoors. School lunches are not nutritionally helpful, but parents have the option of sending their own prepared foods with their children to school if they so choose.
Above and beyond all the critically important factors mentioned above, heredity plays a role in childhood obesity as well. This may be a direct genetic factor, or it may be that children observe parents/role models who are not physically active or who eat inappropriate diets and the children "inherit" similar habits and attitudes.
There are many different causes for obesity in children today. First, children today are not as active as they have been in the past. Second, due to budget deficits in education, PE classes are some of the classes which have been reduced and students are not getting the daily exercise needed. Last, but not least, the diets of children today are not the best. Many schools are on government subsidies and lunches. These lunches are not quite the healthiest for children.