I came across this one here and it was simple enough for them to do by themselves (with a little supervision).
Put 1 cup of water in one glass and 1 cup of orange juice in the other glass.
Then they added 1 tsp of baking soda to the water and stirred it. Nothing happened, they had a little taste and spat it out. Apparently it didn’t taste very nice.
Then they added 1 tsp of baking soda to the orange juice and stirred it. This time it bubbled up and over the side of the cup, making a bubbly mess (they thought it this was great). They were a bit apprehensive about tasting it, but we quite surprised when they did. It tasted like orange soda – which I have to say they haven’t had much off.