The soda tax is a good start because it is easier to separate and a huge cause of obesity in California. Other junk foods are important to target but harder to define. Would Twinkies be junk food if they added oat bran, or would they be considered a source of fiber? The gray area will take time to sift through.
Lets start with sugary drinks and use the money for childrens education, thereby hopefully helping them develop good habits. If that isnt working, other uses could be to offset taxpayer health insurance subsidies or food stamps. It wont be a problem on how to use the money for good. It doesnt have to be perfect to be a smart start.