Add this one to the list of terrible ideas that should be thrown away now, now, now. Basically, the proposal aims to reduce childhood obesity by refusing tax deductions to the parents of obese children, not actually adding a tax. Essentially, it's parent blaming par excellence (e.g. "It's the parents' responsibility that have obese kids"). Moreover, it ignores larger social issues. It's part of that larger problem of individualist thinking that I'm always railing against. It's not about "habits" but habitus. This means that any serious proposal for a solution to rising obesity rates needs to address its structural roots. Ditching the Farm Bill would be a good start. Make high fructose syrup expensive and the pounds will magically melt away.
Oh, and why would we want to raise the taxes of those who are more likely to be poor? Nothing about this makes sense.