To make a homemade soda, all you need is seltzer water (or club soda) and flavored syrup. The ration of syrup to soda depends entirely on how sweet you want it, and it can be adjusted to suit any child's taste.
To make a lemon-lime syrup, simmer 1 cup of sugar and 1 cup of water until the sugar has dissolved, and let it cool before adding 1/2 cup each of lemon juice and lime juice. Mix with club soda and enjoy.
Making cola syrup is a bit more complicated. In a large pot, combine 1 quart of water, the zest of one lemon, one lime and two oranges, three large cinnamon sticks (broken into small pieces), 2 tablespoons of dried bitter orange peel, 2 tablespoons of coriander seed, 1/4 teaspoon of finely grated nutmeg and 1 teaspoon of gum arabic. Whisk until the gum arabic dissolves, then stir in 2 pounds of sugar, and boil for one minute after the sugar dissolves. Remove the syrup from heat, and stir in the juice from the lemon, lime and oranges along with 1/4 cup browning sauce and 1/2 teaspoon of vanilla extract. Let the syrup cool, and then strain and store it.
These syrups generally last up to two weeks when refrigerated.