I agree with TomTom. Use your budget for locks and a ground anchor. No shed, garage or bike storage is that secure so don't depend on thin wood or metal as your security. A scruffy shed built around this badboy in concrete....http://www.chainreactioncycles.com/oxford-terra-force-heavy-duty-ground-anchor/rp-prod8496And a decent chain. No windows if you can avoid it. If you have one extend your house alarm to it, easy if its wireless. Build it big enough for 6+ adult bikes, the kids bikes aren't going to get smaller or less valuable. Big dirty blanket or dust sheet over the lot so nothing shiny is in sight to nosey parkers.
You could always put a ground anchor / bury a big blob of concrete with a shackle under the middle of the shed with a chain link etc coming up through the floor..