One simple way your child can brighten up a shirt is by using three-dimensional fabric paint to pipe on a pocket full of posies -- or a wacky pocket pal.
Slip the cardboard into the shirt pocket. If your child wants to create a pocket pal, have her start by squeezing paint directly from the bottle to draw a nose and fingertips on the top edge of the pocket itself. Next, she can create knuckles and a head topped with spiky hair on the shirt directly above the pocket. Include white eyeballs but not pupils.
Allow the paint to dry for 24 hours, then remove the cardboard and sew black button "pupils" onto the eyes.
If flowers are more your child's style, she can simply pipe on a bouquet of colorful blooms, complete with green stems and leaves that extend above the pocket top.