And after reading the whole thread ...
We just went twice (July and August) with a 6 1/2 and a 4 1/2 (and a 7 month old). I would plan on your kids not making it the entire day at one go. It's possible, but on both of our trips we were able to head back to the hotel mid-day and crash (and the kids did crash ... especially the 4 1/2 year old). Some places to 'crash' at the park ... 50th Movie Theater, Snow White show, Tiki room, Golden Horseshoe,
Disneyland Railroad ... all would give you (and any awake child) something to do while the kids that needed it could sleep. None are really long (although you woudl be able to stay on the railroad a long time if you wanted to), but doing a couple of these early afternoon to avoid the heat and rest might really help.
I am going to re-echo the Fantasyland first thing sugestion. Get there right after opening, and do Peter Pan and Dumbo as the first two, then probably the teacups, then any other ride there that the kids want to do.
From there, it really depends on whether this will be THE Disneyland trip for years, or if you think you will come back in the near future. If the latter, I strongly recommend taking it easy ... DON'T try to do everything (well, you couldn't do everything in a day anyway ... but you get the idea). Since you've never been, you may need to decide if you are going more for the kids or for you. If more for the kids, ask them what else they would like to do (parade, thrill type rides, shows, etc.) and go with that. If more the combo, do some of that, but also let them know occasionally that it is the parent's turn to choose.
There will be very few rides (unless your 4 1/2 year old is short) that will not work for both of your kids ... probably Indiana Jones is the only no go, maybe a couple of others.
As others have said, use the FP and use it effectively (getting them early is great ... you can use them any time after the beginning of the window, even though they put an ending time on it). Also, read up on the child swap on other posts on these boards ... just in case you do have rides that one of the two kids does not want to go on.
Good luck, and have fun!