A good place to start your research might be to look at an encyclopedia such as Wikipedia. It can help you get a feel for the topic, find keywords, and usually has a reference list which can lead to other sources.
Wikipedia (Wikipedia should not be cited as a source in your paper.)
Points of View.
They offer different perspectives on a topic as well as general overviews and can help you to focus your search.
The database Academic Search Premier will provide you with thousands of results, covering many different subject areas. You may want to consider focusing your research on a specific aspect of childhood obesity to find the best articles.
CINAHL is a nursing/health database, that will specifically provide you with medical and nursing journals on the topic, if you're interested in that area.
Again though, searching "childhood obesity" in either of these databases will lead you to thousands of results. I'd recommend clicking on the Advanced Search option in Academic Search Complete or CINAHL, and searching "childhood obesity" in one search box and then another keyword in a second search box, such as "prevention," "bullying," etc.