When I was young(er) a common source of refreshment was lime cordial topped with soda water. This is as much a shorter grown-up version of that as it is a virgin gimlet. It's actually much better than I anticipated and makes me want to try the Mocktini found in The Complete book of Mixed Drinks. Another time, perhaps.
0.5 oz lime cordial
2 oz tonic water
Stir with ice and strain in to a chilled cocktail glass. Garnish with a lime flavoured cherry.