First of all, 20 minute is just too short. Doctors in / of Ontario say kids should get a full our of physical activity everyday at school. That's three times more than what McGuinty proposed.
There's more to this.
He probably didn't do his study before proposing his plan because no 20-minute workout exercise session will help lose weight. Ironically, that's how long of exercise it takes for our body to START burning fat. It uses carbonhydrates, or in simpler term, sugar, for energy for the first 20 minutes. And when its stockpile of the high quality fuel runs out it finally starts burning fat, a much-lower quality fuel in comparison to carbonhydrates.
Think of oil and coal for carb and fat. It's hard to start a fire with just coal, so oil is used to help the process. Until the fire becomes big enough to burn just off of coal oil is needed. That's roughly how our body works.
Giving the children only 20 minutes of exercise a day is just like giving only enough time for oil to start the fire but not enough to burn any coal at all.
If the premier wants to impose physical activity on kids the length of activity should be longer.