Since you are taking two children, four and five, you may want to take a look at the "Unofficial Guide Book to WDW". They even make a seperate book just for taking kids (the first book mentioned also addresses taking children). There are some people (especially on WDWMAGIC.COM) who do not care for the Unofficial guide books. You need to ask yourself: what are your goals? If you want your children to see as much as possible within the time you'll be there, it's best to use a plan (any plan). Do you want to avoid waiting in lines 1 to 1 1/2 hours? Then your best bet is to follow a plan from any of the guidebooks, and use FASTPASS when appropriate. Any plan you decide to follow is only a suggestion for where and when to go next; don't be afraid to alter the plan to fit your family's needs.
I would agree that if your passes allow it, go to MK on the day you arrive. Hit Mickey's Toontown Fair and meet the characters, so your kids can start on autographs and you can start on photos. Take in some of the kid rides, and this will get them started right. This will also help when you go back to MK for a full day, your kids won't feel the pull of Toontown and you can get on some of the more popular rides before the lines get too long.
I don't know if you are aware of this, but I wish I knew this the first time I took my kids to WDW. If your kids spot a souvenir that they want, and it's in a park and the item is themed to a ride or attraction, get it there (providing you were going to get them something anyway). Many items are not carried outside of the parks, not even at Downtown Disney.
Are your kids picky eaters? You can get good ideas on what the restaurants serve at
WWW.WDWIG.COM. It has a great listing of menus for WDW. This will help with any planning.
Use the search feature on this forum to look up any info. You will be amazed how much information is hidden in these threads.
Hope my jibber-jabber helps. Have a wonderful time. Try to post some good pictures of your kids when you get back!