One Day at Disney World's Magic Kingdom - Seeking Tips

Check out Unofficial Guide to Disney World - they come out with a new edition every year and it's a great resource. You might find it to be overkill if you are just going to MK for 1 day (it gets very detailed about all the parks, the water parks, accomodations, etc.) but it does have "touring plans" that you can follow to try to hit as many attractions as possible while minimizing waiting in line. Fast Pass is essential for this. Also, you should get there right when the park opens and be ready to walk in and hit rides as soon as the rope drops - it is not nearly as crowded then as it is later by 11 when more people start showing up. By doing that, we didn't have to wait in line very long for some the most popular rides. Take advantage of their Photopass service too - there are park photographers staged around various locations who can take your picture all together as a family and then save the pictures onto a card that you carry around. Then you just hand your card to another photographer to get more group pics done. After that you can order a CD with all the pics on it. Good luck and have fun!

Pack your own water bottles and snacks. Remember that the drink stands have free ice water (in cups, not the bottles) if you run out.
And use the Fast Passes. Look at your maps and plan: not all the attractions/rides offer fast pass, so find 2 rides in relative proximity to each other, get a Fast Pass for one (maybe send one member of your group with everyone's cards to get them) and get in line for a different ride immediately. (Maybe let the one who went for the Fast Passes join up with you in line). Then, as you come out, you should be ready/almost ready to use your fast passes. As you come out, do the same thing (get fast passes for one attraction, and then go do something else).
You can only have 1 fast pass "ticket" at a time. So you have to use the one you have before you can get another at a different attraction.
Try to hit the super popular ones as early in the day as you can (space mountain for example).

And HAVE FUN. :)

There is a place, right inside the entrance at your right, where you can meet the princesses & Mickey. They are real people in costume, and your child can have in individual one-on-one with each of them. There is no extra cost with minimal wait time. I don't think everyone knows about this yet. Our girls were captivated!!! The best part of the whole day.

Not sure of your kids' personalities, but for my son it would have been really helpful to have him look at the website, see picture sand videos of peoples' visits, and navigate the maps more before we went. It took him the first two days just to feel comfortable with the place, so he didn't really appear to be having fun (although he says he was) until day 3.

Even if you wait until you're there, get a map and plot out where you want to go and what you'll do first etc. Get a fast pass for the thing you all want most, and then deal with lines for the rest.

Go early in the morning to beat some of the crowds and get some park fun in before it gets too hot.

Pack your own snacks.

Make everyone wear sunscreen and a hat or visor. You don't notice how yucky the heat makes you feel until it's too late.

Enjoy. Don't rush to fit everything in. You won't have ANY fun if you're worried running from place to place stressing yourselves out. If you don't get to everything (and you won't) it's a great place to start dreaming your next trip!

Ditto to packing your own water bottles & snacks. Those things are REALLY over priced!

I would go online and make a plan of which rides/attractions you want to experience and which ones are "nice to have". That way you can plan your walk around the park to minimize the back and forth. If you plan to have character photos done, get your park photo pass at the entrance and have it available for the whole day. You can access your photos online for several weeks with the card so don't stress if you don't have time to look at them on the day you're there. Check out the fast pass system for the bigger rides and utilize it! You're welcome to take in snacks and non-alcoholic beverages so I would pack some water and finger snacks to munch while waiting in line for rides. If you've never been, it's a fantastic experience! We took our twin 2 year olds in December and we had a great time. Just be sure to set reasonable expectations and pack your patience.