I was still curious about this heat-setting issue, so went to the source:http://www.goldenpaints.com/technicaldata/fabric.php
There is a lot of good information about this product on this page.... and it turns out you only really have to heat set it if you want to launder it and if you want a softer "hand," meaning if you don't want it too crispy. With kites, that's probably a non-issue. Heat setting is still probably a good idea, though, to make it a more permanent dealie.
Toward the bottom of the page, here's what it says:
• Iron for 3-5 minutes with a medium-hot iron on the reverse side (side opposite of the application). On delicate fabrics where lower heat levels must be used, a longer ironing time will be necessary.
• Professional Oven Cure for 1-2 minutes at 300F; 4 minutes at 250F.
• Clothes dryer: Commercial: medium-high temperature for 20-40 minutes. Household: high temperature for 40-50 minutes.