As my son was only 10 months old at the time, I put chocolate pudding on his hands to make an impression of them on a paper plate (it took numerous attempts to get a useable one) to make a template to use on the shirt.
For older kids, you could probably just put the paint directly on their hands and have them put it on the shirt.
I then filled in fabric paint where the pudding was on the template and pressed the template against the fabric to make the "handprints" on the back of the shirt.
I then used fabric paint and wrote my son's name, age, weight and height on the shirt.
I used a letter stencil and red fabric paint on the front of the shirt to stencil in "Kyle's Grandpa"