in the past, I've backed the squares with freezer paper and drawn an outline around the block the size I want the kids to use. By this time in kindergarten, they'll know to stay in the lines. And you can use crayola crayons which become permanent after heat setting (use a paper towel to protect your iron). You can also use gelly roll pens which come in a ton of colors and average $2 or thereabouts each.