Chlorine dioxide is formed when you drop the PH of sodium chlorite. There are many methods of doing this, including using citric acid and HCl as activators.
The more acid you add, the lower the PH of the solution drops, and more of the available chlorine dioxide is released as free chlorine dioxide in solution.
Your sons face skin is turning red because of the acid he is putting on it.
ml = cc and both are measurements. A metric teaspoon contains 5 ml. Small syringes are graduated in cc increments, and make excellent measuring devices.
The 28% sodium chlorite solution is dangerous to handle, and I have long recommended that people dilute that down to a much safer 5% solution. If you put 11 ml of the 28% solution in a container and add enough water to make a total of 50 ml, you will end up with 50 ml of very close to 5% sodium chlorite.
Sodium chlorite solutions have a certain amount of chlorine dioxide available to them. The amount is related to its concentration. You activate according to your needs.
If you need to only release a small amount of the available chlorine dioxide, and want some left for a timed release of chlorine dioxide, you use something like citric acid . This forms chlorous acid which breaks down, releasing chlorine dioxide in the process.
This type of solution is used to prevent microbial spoilage of meat carcuses between slaughter and consumption, and to keep fruits and vegetables from spoiling between harvest and consumption.
If you need to release all of the available chlorine dioxide, you use HCl as the activator. This type of solution is used for water purification and the disinfection of surfaces.
Some Acne is caused by bacterial infection, so the goal is to kill off the bacteria without damaging the skin. HCl as an activator, in this case, makes much more sense. If a long term product was needed, it would probably be better to simply use sodium chlorite and let the sweat from the pores activate it as needed. This could be included in some form of cream that could be applied to the face.
The challenge in all of this is coming up with a concentration that will be effective, yet do no damage to the skin. I have done a lot of testing, and have found that topical solutions in the 100 - 150 PPM chlorine dioxide range do a good job of disinfection, yet don't seem to damage the skin through short term exposure. The solution I told you about yields a concentration of 120 PPM ClO2.