Hi - Spending the majority of your time at Magic Kingdom is probably the best for the kids at that age. We took our twins in December when they were 21 months old and really did well Epcot and MGM - they loved Fantasmic!. Our time constraints did not allow us to get to Animal Kingdom on that trip. Do the babyswap as much as you can so you can enjoy the rides too!
Fantasy Land in the MK is usually always crowded. You will need to FastPass Pooh, Peter Pan and It's a Small World. When we went in December, IASW was closed for renovation. The girls weren't talking all that much then. We are leaving - again - on the 17th of May and will be able to stay much longer. We've been brainwashing them for a very long time by listening to the soundtracks to most of the rides and having them watch Disney's vacation video showing kids interrelating with the characters! Now, they can sing pretty much all the songs - I can't wait for them to see IASW.
If your kids watch the Disney Channel, don't miss "Playhouse Disney" at MGM - it really is a great show. My girls did really well on Pirates and Peter Pan - we did not do Haunted Mansion with them. They enjoyed the Tea Cups - though Tori did turn a little green and literally fell over onto Daddy's lap. It took all we could to keep the cup from turning after that - Willow loved it. Toon Town is great, too.
On previous trips, you should have no problem seeing most of the AK by early afternoon if you get there for the opening. FastPass what you need to, based on the crowd level.
Have a great trip - and remember to relax!
DH Opening Day Ceremonies 1971
DH & family - 1972 - visit using tickets given at opening day
DH & DW 1983 - off-site
DH, DW, DS (4) 1986 - The Disney Inn
DH, DW, DS (19), DD (12) 2001 - POR
DH, DW 2003 - Wilderness Lodge
DH, DW, DD (15), DD (21 mo), DD (21 mo) 2004 - POR
DH, DW, DS (22), DD (25 mo), DD (25 mo) 2005 - POR
