If it helps, here's our story: our last trip was our kids' first -- at ages 4 and 6. We were up before 5:00am to get to the airport, and arrived in Orlando around 10:00am. The flight was uneventful, and although we had read to the kids a lot about flying ("My First Trip on an Airplane" and "The Noisy Airplane" were bedtime story staples in our house) and were prepared to coach the kids through it, they acted like it was no big deal and like they do this every day.
We were at our resort, checked in and unpacked by 11:30am. I would have loved to have everybody take a good nap at that point (we did take an afternoon nap every day thereafter), but every one of us was far too excited to sleep! So we spent a few minutes "freshening up" and headed to the MK, where we had lunch at Casey's and happened to see the "Move It, Shake It, Celebrate It" parade go by.
Between 1:00pm and 8:00pm (just short of the kids' regular 8:30pm bedtime, which we tried to preserve as much as possible for every night except the last, when we all stayed up late for fireworks), we covered Adventureland, Frontierland and Liberty Square, skipping only Tom Sawyer Island (which we saved for another day) and the Liberty Belle (which was closed for refurbishment). We used Fastpasses for BTMRR, Splash Mountain and the Jungle Cruise, which have the longest waits in those three lands (and stole a second ride on BTMRR during the 3:00 parade!). (*In my opinion, those 3 lands are a great place to start a mid-day tour, since they have a large number of attractions where standby waits are almost always reasonable. Although Fantasyland might have more immediate kid-appeal, it's also going to have a high percentage of VERY LONG LINES by mid-day!) Dinner was a leisurely burger feast at Pecos Bill's. Although it was a long day, everybody was running on pure adrenaline, and there were no melt-downs from the kids (and no requests to be carried anywhere - we travel stroller-less!). Seeing their faces as they beheld the Haunted Mansion and Pirates of the Caribbean for the first time was just amazing, and the big surprise of the day was when my 5-year-old daughter, who we were nervous about putting on BTMRR because we feared it might be too fast or scary, proceeded to scream "THIIIS IS SOOOO GREEEEAT!" throughout the entire ride, waving her little hands in the air. We headed back to our hotel promptly at 8:00pm and were all in bed by 9:00pm, which gave us all 10 full hours of sleep before we had to roll out of bed around 7:00am so we could arrive at a park well before rope drop.
It was an awesome arrival day! I know that a lot of people prefer to relax on that first day and not rush off to a park, but my family and I lack that kind of patience and restraint.