Obviously, if some parents are letting their children, who do not have the knowledge of proper nutrition themselves yet, to eat whatever the hell they want, then yeah, someone SHOULD intervene and give them a proper education and tell them of all the health risks. If, after being aware of the health risks, the parents are still allowing their children to eat whatever they want, I can understand further intervention, but overall, the idea itself is retarded.
Rather than taxing the crap out of sugar laden food items, further nutritional education in the schools. Bring in people who have suffered from morbid obesity, diabetes and heart disease from an overconsumption of unhealthy food choices to talk to the students. Just looking at the kids and saying 'too much of this isn't good for you' isn't going to get the message across.
Ulitmately, it's up to the individual what they put in their bodies, but they should be made aware of all the potential long term risks involved in their life choices. If they decide to take that risk, then that's their decision.
As someone mentioned earlier, regulating alcohol doesn't stop minors from getting drunk. Why would taxing sugar make any difference?