Gel glue or white glue batiking- easy for ages 3-12. Draw on design, allow to dry, paint with watered down acrylic (or spray bottles of fabric dye?), allow to dry. Remove glue by peeling off or soaking in warm water 15-30 mins. Glue dots would make an Australian Aboriginal-inspired look!
Lisa uses simple corn syrup to create beautiful effects on fabric... Sounds like an easy technique with lots of different results depending on what is used to apply fabric paint. Only drawback is finding a place to put the sticky fabric for 7-14 days while it dries. Must try though!