helping sort out a kids project

HI - thanks for adding me to this forum = We are looking to study design and texture during a weekend with some kids. We thought making t shirts would be a great way to teach design, texture and other aspects of art work. We are looking to find an ink which can be used on items found outside IE picnic table wood, tire treads, shells embedded in sand, whatever we come up with. We are looking for an ENVIRONMENTALLY FRIENDLY INK to be used to make the designs on the t shirts. We would need to be able to wash this off without too much of an issue. Next Im looking at how long these t shirts would take to dry, can they be air dried? (if so how long?) must they be in a dryer? (how long?) How do we color fast them? The more information you can educate us with, the better.....Thanks for all your help