We love Ontario Place! Our kids are younger (4 and 6) and we find the attractions perfect for them. If it's a hot day, consider an early afternoon break at the Cinesphere. Imax films are included in the cost of admission to the park.
We usually take our own lunch, but if you want a sit down meal, Marina Grill is good. Another option is a meal combo at Booster Juice (prices are the same as other Booster Juice locations). Selection is limited (some wraps and paninis), but way more nutritious than the usual fast food fare. There's also lots of Pizza Pizza locations in the park.
If you're staying across the road, be sure to arrive when the park opens (10 a.m.). We find crowds much lighter for the first hour or so in the morning than the rest of the day (probably goes without saying for a Disney audience, but worth mentioning anyway

)
There are two main parts of the park - Market Square with the waterpark and some smaller kids rides and Adventure Island with the Log Flume ride, the pedal boats and the bumper boats. Cinesphere and Marina Grille are about half way in between the two areas. We usually park in the parking lot between the two entrances. That way we can go in one and out the other, and just walk across the parking lot to our car rather than having to trek back through the park.
Hope that helps. Ontario Place is one of our favourite family destinations, so feel free to ask if you want more info.