The safety flag is cool idea, I had one on my bike as a child. With my kayak being the Hobie type where you typically use pedal power, the colorful paddle may not quite work.
But lets take a FLL approach and head down to Wally world or order from amazon.com (my choice) a safety flag for a bike for $10. They rise up about six feet and could be added to the kayak with some bent aluminum and a few bolts, washers and thumb screws.
http://www.abikestore.com/Merchant2/...Store_Code=abs
FLL... I attach a whistle to all the kids PFD's and ones used for kayaking,skiing,jet ski, and canoeing. Its a great safety idea for all.