My friend recently had a Onesie drawing party and I had a chance to try out some Crayola fabric markers!
Things I learned:
1) Put cardboard inside the onesie or the markers will bleed through to the other side!
2) You need to put a lot of ink into the drawing or it will fade very rapidly in the wash (even to the point of disappearing completely after a single wash!). I drew all the 'lines' as a series of dots to really make them dark. This was also helpful because the fabric stretches under the marker if you try to draw a line, so the dot method keeps things in place
3) Iron the drawing before washing (set to 'cotton,' iron on the reverse side of the drawing) to help it stay permanent
4) Wash separately (the Crayola website says for at least 3 washes) so that you don't dye other clothing!
5) Buy enough markers! You can't make strong colors if you run out of ink!
6) Stencils are another option if you find some patterns you like!
This is a red-eyed tree frog onesie I made, complete with fake trademark. I drew a black outline of the frog first, and then added in color. The markers bled together a little, so there are some areas near the black lines where the color gets darker, but for the most part they worked well. You can see in the blue areas that I was running low on ink so I couldn't get the color quite as strong as the green and orange.
This is the Crayola site regarding their fabric markers:
There are other fabric marker brands out there, I just haven't had a chance to use them yet! I've drawn on shirts using permanent markers before (definitely wash separately the first few times!) and they do fade after a number of washes, so I'm hoping that using real fabric markers will mean greater longevity!