We just took our three year old daughter to Disneyland last April (along with my four year old son) and had a blast. Some things I wish I had done differently? For one, I decided to pass on bringing our stroller or renting one because I assumed it would be more hassle than it was worth. But when my daughter fell deeply asleep from sheer exhaustion at 7pm, I was sure wishing I had someplace to put her. One of us was forced to carry her limp little form around the park until it closed at nine...we had to get our money's worth, after all. ;-)
I would suggest renting one ($15 for the day) as opposed to bringing your own so you won't worry so much about it getting damaged or stolen.
The only other thing I can think of (and possibly the biggest mistake we made in regards to my daughter) was not taking her to meet all of the Disney princesses FIRST THING when the park opened. I'm not sure if they have something similar at Disneyworld, but there is basically this setup where the kids wait in line to meet (and hug) four or five female Disney characters in full costume and have pictures taken. It was the one thing that I was SURE my three year old might possibly have a chance of remembering later on in life.
But apparently it's also one of the most popular "attractions" in the park and the waiting time during the rest of the day is fairly prohibitive...over an hour at least. And there's no "fast pass" for it, either.
So for your daughter's sake, if there is such an attraction at DW, please make a beeline for it right when the gates open.
Oh, and as for the time of year, I've always heard that winter was their slow season. So a Jan/Feb trip should be pretty ideal.
EDIT: I just noticed someone else had mentioned that one parent would be forced to wait with your daughter and miss the good rides, but that's not true at all. Not anymore. They have something now called Child Swap which is basically a means for all three of you (mom, dad, child) to wait in line for a ride together. Then, when it's your turn, one of you goes on the ride while the other one is escorted to another area where people exit the ride. There, your spouse will meet you, take the child, and you will be allowed to enter the ride immediately through the exit side.
It's especially great if you're in a group, that way you can split up to where you and hubby can ride together and then your boys can ride after. Or however you want to work it....
More info at: http://www.orlandofuntickets.com/Child-Swap-at-Disney.php