I have to say this is my favorite park in Toronto. It's huge, has nice trails for you to trek through, and beautiful sights. It's also very quiet despite a large number of people who visit. It's a great, quick escape from the city. My only beef with the park is the zoo. It's very small and doesn't seem to be kept well. The animals seem hot and unhappy. When you go to the Toronto Zoo, it's fun and interesting (though admittedly I feel bad for those animals as well), but the High Park Zoo is quite depressing. I suppose if your kids like animals and don't care which ones they see, it's a good stopping point.
Overall, the park is large, clean, and there are multiple things to do. My suggestion is to take a picnic basket and blanket there some Saturday/Sunday afternoon and get some much needed vitamin D. Make sure you have some cash on your for some ice cream!