In a saucepan, heat sugar and water to a boil, then simmer for 2 to 3 minutes. Add the lemon and lime juices and cook 1 minute, then add the lemon slices, if desired, and cook 1 minute.
Transfer mixture to a 9- by 13-inch glass pan and freeze for 2 to 3 hours, or until half frozen. Using a fork, break up the ice until texture is almost smooth. Freeze for another hour.
Break up ice crystals with a fork and scoop mixture into a parfait glass or tall, narrow glass; serve with a spoon, a straw, and, if desired, a slice of lime on the edge of glass.
How kids can help: Scrub fruit thoroughly; squeeze juice from lemons and limes; break up slushy mixture with a fork.
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