Could it be that in remembering this past you are not debarred from the blue remembered hills, but rather, you are inhabiting them for the very first time - inhabiting them in a fuller, realer way than when you were there as a child? Memory, though conflicted and anguished, affords you a vivid new presence (and it may also be that Potter wants us to superimpose an ironic meaning on "land of lost content" with its "air that kills"). Now you are there, really there, intensely aware as you never were at the time of the ironies, the injustices and the exquisite luxuries of having nothing to do all day.When an adult truly remembers what it was like to be a child, with an adult's perspective, there is something forbidden and almost transgressive about it. That is what, I think, Potter is getting at when he cast adults as children. It wasn't a stunt: it was a representation of the act of memory.