We've eaten at The Grenadier a few times this year and it's been pretty good. Definitely feels like you're eating at a city-run cafeteria but overall, it fits well into High Park and the food is pretty good and very reasonably priced. You can self-serve or have table service (which we opted for). I find the indoor part very dark and in need of a facelift.
However, the last time we were there, my son ordered a hot dog and fries from the kids menu for the first time. It was definitely not kid-sized and definitely not geared towards kids. It was a HUGE hotdog... I would even say it was a sausage and it had a very heavy and strong smoky taste to it. Not what I imagined it would be and definitely not attractive to a kid. Needless to say, he had one bite and ended up eating the fries for lunch... and it's not that he's a picky eater....he was hungry and loves hotdogs. But real hotdogs aimed at kids not this huge strong tasting thing.
Also... it was a gorgeous day outside so we sat on the patio which was infested with bees (or wasps? I don't know the difference...either way, take warning - it was really bad!). Our table was in the sun near the fence with flowers and it was particularly bad; to the point that there was no way we would be able to eat there were so many of these little buggers attacking us. As our lunch arrived, I asked to move to the upper deck into the shade and was met with a snotty response from this teenage girl who worked there. She said "they're everywhere" and grudgingly moved us. Funny, we didn't encounter any of these pests in the shade. I had to ask her for cutlery for our lunch which she brought along with her attitude. I've noticed before that service is hit and miss here and today, it was definitely a miss.
Overall though, I would return... it's a good place to have breakfast or lunch in a beautiful park.