“How can obesity be prevented in children?”

Yes by having a very good habit of doing exercise every day if possible and following a healthy diet. Obesity is preventable but has to start at home by educating them on the consequences of it. There are things that are learned at home not at school or on the street. Remember, children are reflections or images of what parents are. Children should have at school, as part of the regular curriculum, physical activity as mandatory, except for those who have specific health reasons for not participating.
Perhaps you belong to the baby boomers generation when school made you to participate in sports as part of your education. That generation didn't have as many obese children as today.
Therefore, stop given electronic devices to your very young and young children but if you do impose rules and pushed them to participate in a sport at school or in your community.
Make your self an example, if you are obese, by exercising and following a healthy diet. Your achievements will be the inheritance of good health for your children.

It starts with what we eat and setting a good example of what foods we eat as parents as well. I am a big believer in not asking our children to do something we are not willing to do. It is important with children to be careful with candy, chips, pasta, wheat bread, etc. They should eat meat, green vegetables and keep the fruit to raspberries, blackberries, strawberries and avocado. The other fruit has too much sugar in it. You can also work with a bariatrician in your area to help. To find a trained bariatrician go to: https://obesitymedicine.org/find-obesity-treatment/ and you can have a professional work along side you. I have treated thousands of children/adolescents in my career and it is a pure joy to see them get healthy and feel good about themselves during this critical time of life where self-esteem is developed.