How about we look at it like this? This is a problem you can't do anything about directly. You can educate, and attempt to set a good example, but you can't intervene. You can't introduce the prospect of the government taking children away for obesity.
A likely response is "Well, we already take children away from parents who are abusive in other ways, this is abuse that is arguably just as bad as those others".
First of all, no it's not. Letting your child get fat will never be as bad as directly abusing them emotionally or physically. The child is simply being allowed to engage in an activity that they find pleasurable, but has some consequences. They're not being raped, beaten, or likely even emotionally assaulted by the parent directly. Any emotional problems will usually come as a result of the cruelty of their peers. There is some element of negligence here, but its not bad enough to ever warrant taking the child away.
This sort of justifies the use of the slippery slope argument as well. We got on this slope a long time ago, and if something like this passes we will have slid just a bit further down it. It's happening right before our eyes, and just as our gut has always told us, each step is easier to take. The only way to stop is stubborn refusal to go farther down this path. Accept that this is a problem you can not solve through direct intervention because direct intervention is wrong. Also, using something we already do as justification for going even further is the worst thing you can possibly do to argue your case.