Primary sources are source materials that are being studied. The term refers to "an original document, source, or text rather than one of criticism, discussion, or summary" (Oxford English Dictionary). In the context of children's literature and childhood studies, primary sources can be texts and images for young readers, objects and materials used by young people, a children's author or illustrator's archive, etc., depending on your research questions.
Secondary sources are analytical or critical commentary on primary materials. These are existing scholarly publications you will synthesize and build upon to develop your own thesis and argument.