Master and Crazy Hand: Kid's hands
Master Giant: The kid trying to destroy his toys
Master Beast: The kid's dog maybe
Master Edges: The kid using sharp objects
Master Shadow: His other toy version of the character
Master Fortress: The kid's...stomach? It even has a beating sound while going through that form which is his heart
Master Core Orb: I have no idea....his soul? I really don't know
I may be wrong.