Begin by getting your hands on the Lego rock monster toy. I'm guessing if your kid wants to be this you already have a few. Then cut out and form a cardboard tub to fit your child. Determine the amount of space needed for the head opening and then cut a Styrofoam ring for the top to hold the cylinder shape.
Draw on and cut out the hands from the foam. Stack 3 sheets high and cut out area for pringles tube. Hot glue pieces together and repeat for other hand.
Cut to size and hot glue a cardboard insert pieced for the hands. This adds strength and helps look more like the toys hand. Roughly shape the hands to look like the toy.
Cut out a few circles and glue together to start building up the head piece.
Take measurements of your child's head and build up as needed.
I shaped out the inside of head piece and lined it with cardboard strips (keeps Styrofoam out of the hair and looks cleaner)
Rough cut rock looking shapes using a Dremel or other tool. This is a good place to use the toy for a reference.
I recommend reinforcing the inside mouth opening with cardboard strips and then cut out teeth shapes.
Stack and glue up 2,3 and 4 high curved pieces of different shapes for rocks. Sand at different angles like the toy. Where a mask and glasses because this foam can't be good for you to breath and it gets everywhere.