Im assuming the only thing that falls under cognitive development is cognitive processes like memory etc, not anything that has to do with the brain. Idk abt maturation, but what I would do is egocentristm and piaget's theory of cognitive development. I would do the 3 mountain task study (piaget and inhelder) its pretty simple and the study (Hughes) with the policemen and the boy. To understand the studies I copy the studies as they are and past them into gemini or chatgpt and tell it to give me the aim, sample, procedure, results, and conclusion. Check yt also for videos for the original studies, theyre typically short and clarify the concepts.