Position a rack about 4 inches from the heat and preheat the broiler. Cut grapefruit in half and trim a thin slice off the bottom of each half, if necessary, so that they stand upright without wobbling. Using a small knife, cut along the sides of the segment membranes to loosen, but leave the segments in place. Remove any seeds. Refrigerate 10 minutes.
Arrange the grapefruit on a foil-lined baking dish. Help the kids spread 1 to 2 Tbs. sugar on top of each grapefruit. Transfer to the broiler and cook until the sugar bubbles and melts, 2 to 4 minutes. Watch closely to avoid burning. Remove from the oven and let the kids carefully sprinkle with the toasted coconut.