Start with your textbook to begin identifying parts of your answer to the assignment's guiding question: Why is play important to the development of children? (Focused on PreK - 3rd grade)
Textbooks provide helpful overviews of topics, making them a good place to start gathering ideas...plus, you already have it lying around. Convenient!
You know what points you need to cover in your paper, so start taking notes in an outline format to organize your research. This way you'll know what you know -- and where the gaps are that you need to fill in with additional sources.