Let plain donuts sit out overnight to harden. Put vanilla frosting in a bowl, add a few drops of gel food color, microwave for 10 seconds and stir. Dip one side of the donut in frosting. Add sprinkles and set out to dry. Loop string through each donut and drape over a support so you can adjust the height as each kid steps up to take a bite—no hands allowed! Tip: Hang the donuts barely within tippy-toe reach.