From the moment I gave my 4 year old son some of my old 2nd edition plastic Space Marines (painted up as Blood Angels), he's been obsessed with my 40K models. He also constantly begs me to draw pictures for him that he can color.
I finally caved in and decided to give him more than a quick sketch. This took roughly two hours, penciled & inked by hand (can't remember the size or brand of the pen I used - something from Michaels Arts & Crafts), with a few minor tweaks in Photoshop to clean it up (I suck at inking).
I put two images onto a single page and printed several pages for him; he's been on a coloring bender ever since.