Hi, a warm welcome to the site. There's information on how to begin, undertake and evaluate your study in the
level 3 handbook - pages 383 - 394
Your tutor will also explain how what other sources you'll need to research, and how it's possible to look critically at the information you've collected so you can see differences eg. if the two children are the same age their skills may differ in the area of development you've chosen. These differences might be explained through the work of the selected theorist and enable you to find ways to support each child so they can progress.
Hope this helps a little, best wishes