Possibly rig a pack to the back of the wheelchair or "saddle bags" to the side of the walker. Another option would be to use a small cart that supplies and kiddies can ride in and dad can push/pull. Or if the wheelchair is motorized set up the cart to be pulled by it. Not good for wilderness, good for urban environment. Not that I forsee bringing two sub 5 year olds and a wheelchair/walker bound mother into the wilderness as being an overly viable option.

-Bill Liptak