Inktense has to have moisture from fabric medium or aloe to moisten it. It goes on like colored pencil but turns to permanent ink when wet. Check out YouTube for examples. I just used them on a Christmas stocking and they are fun to work with.
One caution...they can bleed past your lines so could seep past your embroidery. You'd need to practice first. And use fabric medium rather than aloe. Maybe let everyone color but you be in charge of the moistening part. Also if Inktense touches the embroidery it might change the color of the thread. Again practice before doing your project.
Standard Crayola crayons work great. Anyone any age can color. Press hard to get deeper colors and iron blocks between paper towels to set the wax. You can color/iron again to deepen a color if needed. I had my grandkids draw and color on fabric that I made into pillows for their mom and it turned out great. Wash and dry on cooler settings. It might fade over time if the quilt is washed constantly but should hold up pretty well.