It is my opinion that Miracle-Mineral-Supplement works best when taken with water. You can always add more water to dilute the solution to the lower the concentration to where it is palatable.
However, after activation, adding Sprite should not mess things up too much. citric acid is added to Sprite to give it a "tart" taste. I don't think there is enough citric acid in Sprite to activate a sodium chlorite solution, but I haven't actually tested using Sprite as an activator.
You should consider recommending splitting the 8 drop dose up into two 4 drop doses, and diluting the 4 drop dose with 250 ml (about 8 ounces) of water. That would drop the concentration of ClO2 down to where it should be more tolerable. Using 500 ml (16 ounces) of water would be better for a 4 drop dose, but some people prefer a stronger solution.