Being able to design and actualize your own fabric is an amazing capability. You can design fabric and send it to Spoonflower to get professional quality, but if you don’t need large quantities and your pattern is fairly simple, there are other options. Like fabric markers!
Alicia of Dismount Creative created a bright triangle print fabric using Sharpie fabric markers, and then she made a little pouch out of it. It would be neat to let a kid draw fun designs on fabric and then sew up pencil cases or tote bags for their friends.
DIY Geometric Print Fabric (and a Pouch That Was Made From It)