We have been taking both our DDs since they were 1.5 years old. Youngest will be 4.5 when we visit this July.
We always follow a plan, bypassing rides to return to favorites at a later time. There is always so much going on, that redirecting their attention has never been a problem.
For the ability to ride a favorite ride several times in a row (in say, Fantasyland) we find it best to take advantage of Early Entry and head straight there. We have been able to walk on Dumbo a couple times in a row, several trips in the tea cups, etc. We always finish Fantasyland completely with no wait for anything. This even works fairly well on non EE days - just get there RIGHT when the park opens.
As others have suggested, Fast Pass is your best friend. We always send someone (ME!) ahead to get the next set before using the current one. By using fast pass and getting to the parks right at opening, we have NEVER spent more than 15 in line (typically, 5 - 10 minutes, max or a direct walkon.) This even worked great this last trip during Christmas.
The 4.5 yo WILL need a stroller. Our 6 yo still needs one sometimes - and its easier to get them to the next attraction when you are the one pushing them there.
All that being said, do try to break away from the plan at times. These end up being some of the most magical moments. Our two love to stop and see the characters. When you are in a hurry this can seem distracting, but it is a huge part of the Disney magic, and makes the stay that much better. You will NEVER do everything, so just plan out your favorites, and then hit the rest if possible.
have a great trip!!