I definitely agree with taking a break in the middle of the day. For us, that was sometimes possible, sometimes not depending on what park we were in and what was going on. But, on the days you do that you will notice a BIG differene in the attitiudes of you and your children for the rest of that day and likely the next day. WDW is a lot to take in, so kids (and adults) get overwhelmed quickly. Taking a couple hours to "chill out" and re-set helps a lot.
I very much agree with getting to the parks when they open. You will not regret it. You will walk on rides and get fast passes so much easier. I just can't stress enough how much it helps to get there early.
Buy ponchos in advance. Without fail, it will rain on your trip. Yes, they sell ponchos in the parks - but they are ridiculously priced and don't last long. Bring your own. Have them with you at all times (well, defeinitely in the afternoon). AND...when it's raining - ENJOY IT!! It's a warm rain, so you can be outside and not be freezing. Go on rides when it's raining! People tend to hide in stores and restaurants during the rain, but the rides are still running and there's no lines. When my daughter was 3 we went for a week and it poured every day - we had a BLAST! Yes, we were soaked, but we were never cold. It was really fun! Don't be afraid of the rain!
Remember, your tip is mandatory for sit down restaurants and it's not included in your Dining plan. Just don't be shocked when you get your bill and you have to pay for the tip. Eating at nice restaurants every day means those tips add up quickly. Same story for alcohol- if you want a glass/bottle of wine, you're gonna have to fork over the cash
Get a spot for parades early. Your kids are young enough they will probably enjoy the parades at MK - well at least the night parade. The spots go quickly. We like to set up shop along main street right outside the Confectionary so we can take turns leaving a getting snacks

Then after the parade, we walk into the middle of main street and have a great view of hte fireworks, adn then when that's over we miss the majority of the mad dash for the exit. HOWEVER, if your kids are up for it (i.e., not fallign asleep during the fireworks), you can stay in the parks for a little while after the fireworks are over. You don't have to leave rigth away. You can get on some rides or haev some ice-cream. Then you'll get to hear the MK Kiss Goodnight - It's pretty magical!!
Okay - enough for now! Enjoy your trip!