Child obesity is a decades long trend, it's independent of political party.
There are contributing factors you might be are unaware of, as its not simple as people being lazy, or because of work culture, as dmgtz suggests. The US has diverse range of chemicals in its food and that other countries do not. Well over 2000 ingredients that are banned in Europe are available in US, which is ridiculous. Private companies should not be allowed to put harmful things in food. The FDA allows this because of corporate capture and weak governance.
Some of these chemicals work through cellular pathways and damage the mitochondria, which is responsible for the metabolic health of an individual. They literally cripple the ability for people to process energy correctly.
Even the endocrine system (hormones) plays a huge role in weight management, ask anyone with a thyroid issue. Glysophate aka Roundup is a common chemical sprayed all over the food - CDC says 80% of people have it in their urine. It is heavily linked to thyroid disruption among other horrible things. It was RFK who took on Monsanto, won three court cases exceeding a total of 2 billion dollars worth of payouts [
1] and who directly led to the removal of Roundup from garden centers around America. Probably hundreds of thousands of lives impacted by this, maybe more.