The "work back to front" strategy is an old Birnbaum Guide hint from the pre-MGM/AK days. I don't think that I've seen it mentioned anymore. It has gone the way of "always take the line on the left because people tend to line up in the middle and on the right".
If you have a map of the MK, you'll see that no matter what, you've got to go down Main Street to get to the various "lands". Not many people do their shopping on the way in anyway. The various "lands" are spread around a central hub (the castle). You can choose to work clockwise around the hub, starting in Adventureland or counterclockwise by starting in Tomorrowland.
With a 3 YO, I wouldn't do either. Instead, you may want to head straight to Fantasyland and do all those ride first before things get too congested. For some reason, that land gets bottlenecked before noon. After Fantasyland, head over to Frontierland or Tomorrowland, which ever one is closer. From Frontierland, go to Adventureland and then cross in front of the castle to Tomorrowland. From Tomorrowland, cross in front of the castle and continue to Adventureland and then Frontierland. I would definitely avoid taking a "shortcut" thru Fantasyland. It won't save you any time.
As far as driving/parking vs. Disney transportation is concerned, consider this: If you drive, you will have to park, take a tram to either the monorail or boat and then take the monorail/boat to the MK entrance. If you take the boat or bus, you will be dropped off at the MK entrance. This is not the case for all of the parks but it is true of the MK. For all other parks, the buses drop you off at the entrance to the parks. You will need to either walk from your parked car or take a tram to the entrance.
