My favs for my super active kiddo are miniboden. They're not cheap at $36, but they sure stand up. ((Hint; after your first purchase you get put on the "deals" list. Buy 3 get 1 for a dollar, 25% off your order, free shipping, etc. So if you do order from them, make your first order a small one))
http://www.bodenusa.com/en-US/Boys-Clothing.html
http://www.bodenusa.com/en-US/Boys-Trousers-Jeans/Jeans/2...
We buy his a size big (they have double 1" thick elastic on the inside that buttons on each side to different waist measurements, and cuff the bottoms) and they last 18mo -2 years of knee sliding, skateboarding, tree climbing. We've had 1 pair rip out of the 10 or so pairs we've bought over the pas 6 years (since he was 2). All the seams are reinforced, fabric is reeeeally durable. Toddler sizes have doubled fabric on knees, but kid sizes don't.