Just going to go over a couple of your points here:
A) Disneyland is something Walt probably couldn't have afforded when he was a kid ... even at the prices Disneyland was charging when Walt was alive (so no WWWD).
B) You say in your post that most of these kids have gone to SFMM and Disneyland, but not museums. Last time I checked, most (read ALL) museums cost less $$ than SFMM and Disneyland. This is a matter of PRIORITIES, not finances.
C) There ARE museums that are priced less than the $20 charged by the WDFM, some were already pointed out to you. The Getty in LA is actually FREE (you only need to pay for parking or take the bus to get there). I wonder how many lower income families have taken advantage of that? The LACMA is free for children, $15 for adults. So $30 get's a family of 4 in there ... cheaper than a movie.
D) Speaking of movies, I'm sure many of the lower income families you speak of make it a point to go see the latest 3D cartoon or Transformers movie. Those tickets are at least $10 a piece, if not more (for 3D).
Again, it's all about priorities, and parents these days seem to be keen on doing activities other than museums. So the museums have to price accordingly for the people who do visit in order to keep their doors open. Just the way it is.