Osprey have a big pack.
I personally use a W.E. Lost World
http://www.wildequipment.com.au/backpack_detail.php?Code=WELOST which is similar to the W.E. Mountain Expedition.
Additionally to that I have bought the Aarn expedition front pockets with the hope of sewing my own retrofit connections to my hip belt. I also own the standard W.E. side pockets and also the back pocket. Alas, the additional back pocket doesn't actually fit the Lost World.
Other advice I can suggest is that you should try to get the W.E "Long Side Pockets". They are an item that I doubt any retail asstistant would ever suggest as they are almost an enigma. There is only a small mention (and photo) of them on the WE page.
http://www.wildequipment.com.au/backpack_detail.php?Code=WEWCS click "
"Long SIDE POCKETS
The WildChild can be fitted with long SIDE POCKETS, supplied in pairs. Make sure you order the size corresponding to your pack size: S/M or M/L. They mount on the side compression straps at the top and, at the bottom, on the webbing tabs provided. These long pockets are idea for carrying the clothing bulk needed for young children. They also provide extra side protection to your child's legs where they protrude normally from the seat compartment.
Canvas side pockets:
For OLDER MODEL WildChilds. For multi-day walks these full-length, L-zipped side pockets provide the extra capacity required for clothing and other gear. They also provide excellent side-protection for the child's legs. These black canvas ones are equipped with our original overlapping press-stud straps at the bottom end. These pockets suit WildChild packs equipped with short, vertical webbing pocket mounting tabs on the bottom edge of the lower compartment sides. A pair of M/L pockets 550mm high x 150 wide x 100 thick, adds almost 18 litres capacity to the pack. A pair of S/M pockets, 50mm shorter, adds about 16 litres.
Nylon side pockets:
For NEWER MODEL WildChilds. For multi-day walks these full-length, L-zipped side pockets provide the extra capacity required for clothing and other gear. They also provide excellent side-protection for the child's legs. These Oil Blue 1000d nylon ones are equipped with webbing barbs on the bottom edge. These pockets suit WildChild packs equipped with rectangular plastic loops in the base seam of the lower compartment sides. The pockets are 150mm wide and 100mm thick. A pair of M/L pockets adds almost 18 litres capacity to the pack. A pair of S/M adds about 16 litres."
