My twin girls were 21 months in September when we took them for their first Disney Vacation. We also have a 5 year old son, this was his 3rd visit. We planned on keeping things at a slower than usual pace, and they soaked up and loved everything. Any ride they could be on, they were, of course Dumbo and Magic Carpets were two of their favorites. We were at Disney 9 days and stayed at the Caribbean Beach Resort (the girls loved the pool, it was wonderful for children of any age), however; we had extra help from an Aunt and Grandma, and when you are trying to get on the buses you need all the help you can get. This also allowed my son and husband to go do to the more grown up rides, etc. We didn't go to Animal Kingdom, but enjoyed the other 3 parks, multiple character meals and even went to Mickey's Halloween Party.
As far as remembering the vacation, the girls just have to look thru one of our photo albums or see the pictures on the computer and then they can tell you each characters name, the castle, and of course they say they want to go back. Each trip is what you make it, no one else can determine the amount of fun and not so fun you will have, so go and live it up. Enjoy!! The pictures are priceless!!!