So take your material and cut out a section off one end for your chaps
Now for the fun part, get your hot glue gun ready!!! You are going to hem up the edges of your chaps (all but the outside edges) with your hot glue gun. Put a line of hot glue along the edge of you chaps and fold over press down and let dry.
Take on of your child's shirts so it can guide you for cutting neck and arm holes fold your material like it would be a vest on over the shirt and place the shirt on top of the material. You want to material to actually be a little bigger than the shirt.