We found some old t-shirts that had some paint stains already on them. Seriously we have many articles of clothing with paint stains on them.....who wants to stop and get an apron when the the creative juices are flowing?! Luckily they were shirts in fall colors! We just used regular acrylic paint. I am sure it would work MUCH better with fabric paint.
Acrylic paint is harder to paint on fabric, but it stays on just as well. I have acrylic stains on clothing and it never comes out in the wash-- ever.
The boys are not quite finished with theirs- we needed to let them dry a bit more so they will be able to add more details.