With little ones, I would head directly to Fantasyland and plan on being there for a good while.
Here is something to keep in mind--you won't be able to get through the entire park in one day, so just look at it as a "sampler" of visits for the future and enjoy the quality, not quantity.
Worth renting a double stroller, too. It will keep them contained for getting through the crowds and provide a spot for a cat nap when parked in the shade. It'll save you wear and tear since you won't have to pick them up as much, either. Also serves as a backpack holder as you move along to give your shoulders a rest. Leave something sticking out of the back pocket of the stroller when you park it or tie a bandana to it to make it easier to find later on. Folks are less likely to accidentally grab yours that way, too.
Get Fast Passes for your group for your favorite ride right away (grab the map that indicates which rides have them as you enter the tunnel after the entry turnstiles--it's also the stroller rental area to the far right) when possible, too. That'll save you time in lines.
Here is our favorite respite for an early lunch-- Pinnochio's Haus in Fantasyland. You can sit in air-conditioning, eat whatever you may have brought with you, and have the option buying of sides, drinks or meals sold inside.
It's a Small World is a good attraction to catch with little ones if it's open-- was closed in December for renovation. Long lines move quickly through it.
If you get tired of waiting in lines later on, you can use the sort-of-hidden sidewalk that takes you from Fantasyland/Toon Town (back of race car area) to Tomorrowland. I'll let you in on our secret fave for re-energizing-- the Tomorrowland Transit Authority (TTA) ride. Rarely a long line as it moves along so well, you go up to the cars and jump in. Takes you through the interiors of the major Tomorrowland attractions and is a little ride in itself. It's awesome when it's hot out or raining, too, as it's covered and goes thru AC. Be prepared that one part is dark-- it goes thru Space Mountain- but tell them they'll be seeing "nightime" at Disney and to look for stars and the people on the roller coaster! The other easy-to-get-into attraction is the Carousel of Progress, across from TTA. You sit in upholstered chairs, enjoy the AC and watch an animatronic show. My 8 year old DD has been hooked on it since age 3. In December, I just shut my eyes for a few minutes to relax as DD and DH watched it for what had to be the 20th time in 5 years.
One last thing... keeping everyone hydrated, rested and fed is a huge factor in making the day go well. Bring along snacks in ziploc bags (keeps things in when they drop them), pre-chill juice boxes (bring along a softsided lunch box for these items-- stuff it into a backpack) and make then wear hats/visors in the sun. Splurge on a Powerade later on. They have twist tops and will get liquids back into you quicker than water-- only because you'll drink more since it goes down so much faster than water!