An adult should cut the watermelon in half. Child may use an ice cream scoop to remove chunks of watermelon and place in a glass 4-cup measure. Transfer 4 cups of watermelon to 9- x 9-inch glass or metal baking pan. Try to remove as many seeds as possible. Child can use fork to crush watermelon and get the mixture as smooth as possible. Mix in sugar and lemon juice. Place dish in freezer and let mixture set for 30 minutes. Pull it out and have child mix it with a fork, then return to freezer. Repeat scraping and mixing the Granita every 30 minutes for about 3 hours, until all Granita is crystallized. To serve, scrape mixture into serving dishes with an ice cream scoop.
When shopping for a watermelon, look for one that is heavy for its size, has a bit of yellow on the underside where it was resting on the ground as it grew, and a stem that is dried up.
Wear clean gardening gloves that have rubber grippers on the palm when cutting watermelon. It will give you a good hold of the melon and help prevent accidents.