The majority of todlers are stinky dum dums that eat poop and scream. However, assuming this is somehow a real question, my best guess would be that using crayons to color is much more simple for them, and also less dangerous. Using pencils, markers, or pens can easily stain stuff or can be used as a small shank. Crayons are.. well.. have you ver tried to stab someone with a crayon?
