Image 1 shows a printout I made from my printer. This is an A3 image because my Canon ix6860 can print A3 sizes.
Measure the child’s head or simply go for an average size.
You can with the child paint, use crayons, sketch, draw, any image you want on the hat, such as a big 5 for a birthday party for someone who has just turned 5 years of age.