No, no it wouldn't. It would be more effective to stop subsidizing unhealthy food. The long-term effects of such legislation are almost guaranteed to be a net-negative because of edge cases. There are already way too many laws. You should always look first at what can be removed, rather than what can be added.
It's somebody else's issue, and I refuse to accept any responsibility or worry over the fact that somebody else's kids are fat. I have enough problems of my own, I've got better things to do than accept part-ownership of the easily-soluble problems of others.