Panorama Ridge is beautiful and the Garibaldi trails are smooth walking, but this hike is likely a bit long for the kids (15 mi?) and Garibaldi is always packed out with H. sapiens in the summer. Also if you're only doing one hike in Garibaldi it's a bit of a crime not to do the Black Tusk, but again that might be too long for the kids (and too sketchy if you go all the way) and it's busy.
Similarly Wedgemont Lake is a beautiful but busy hike. A lot of the Whistler area stuff is busy since it's such a destination. Joffre lakes is about an hour drive north from Whistler (beautiful drive) and the time commitment is a bit less, but it's still going to be packed out in the summer. All four of the hikes I just mentioned are beautiful but busy.
There's no decent trails around Whistler that aren't well used in the summer, but Rainbow Lake is one of the least busy areas. It's not quite as scenic as some of the aforementioned trails but it's still great and feels more like an escape.
That website above has a really good listing of the Whistler area hikes. It's a good site because it's most comprehensive. Shoot me a PM if you have any questions as I've done most of these hikes (I lived in Whistler from '08 - '12).