As far as eating goes, you can't really dictate quantity but I think one thing you can do is subsidize healthy foods and make them cheap and affordable so everyone can have access to them. I know if you grow up poor you just can't afford to buy healthy foods b/c they're too expensive.
As far as physical activity is concerned, we've had unhealthy foods for a long time and people have eaten unhealthily since the 50s. Look at this chart: http://wellandtrue.com/the-world-has-changed-so-have-our-wai... and you'll see that we only start getting a big spike after the 1980s when everyone basically started moving to fulltime desk jobs in front of a PC all day. So I think some kind of nationally mandated PE routine kids had to do M-F would be a start even though it'd probably never get support.