Take a T shirt.
Cut it up. I used a pattern from "Weekend Sewing" by Heather Ross. I've actually used that particular pattern several times, for pj's for me and the kids, and several re-purposed pants for the boy-child.
Don't even worry if the child's legs are long and you have to use the entire length of the shirt and go up into the arm holes. It makes a cool pattern on the sides of the pants.
Show them Off!
Yay to new pants! It's like wearing pajamas all day long!