Add the baking soda, 1/4 teaspoon at a time so you can watch the reaction twice. Give it a good stir to really get it going!
Mix together the water and the sugar in a separate measuring cup, and then add it to the fizzing lemon juice.
Notes
I wouldn't recommend drinking more than a 1/2 teaspoon of baking soda at one time, so don't drink more than one serving of this lemonade in one sitting. If you are giving it to kids under 5 years old, make sure you split it into two glasses when you are finished, so they aren't drinking more than 1/4 teaspoon at one time. It's not the end of the world if they drink the whole 1/2 teaspoon, I would just try to avoid it if you can.