So much depends on the gender, interests and personalities of your kids!
We went last fall for the first time, with DS6 and DS2. DS6 was nervous about meeting characters until I told him they were just people in costumes. (I did warn him not to mention this to anyone else, since we didn't want to ruin it for others, especially DS2.) Thought he was OK then, but when we got to the park and met our first few characters, he refused to meet them and get photo/autographs. He knew I was going to make him his very own scrapbook, so I said casually, "Gee, I don't know what I'm going to put in your book. But DS2 is going to have LOTS of pictures and autographs in his." (Yes, it did work, and he also seemed to forget our little secret when he was 'in the moment'.

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DS2, on the other hand, vaulted himself into Goofy's arms when we saw him right after check-in at the DLH. He was the one we thought might be afraid, but it was quite the opposite.
Thought the kids might be worried about dark rides, even the tame ones, but no problems there. In fact, DS6's first ride was Splash Mountain, against my better judgment, but this turned out to be his favorite ride. DS2's favorite was Winnie the Pooh.
I realized, one hot afternoon as we were standing in a long line to meet Ariel, that we seemed to be the only ones with little boys in line to meet princesses. I felt slightly guilty, but my boys didn't even seem to notice, and in fact the princesses seemed to give them extra attention (perhaps because they are a rarity.) They loved meeting, talking with and hugging the pretty princesses!
I understand the menu for children has changed a bit since our trip (to offer healthier sides and desserts.) We did let our boys choose their own meals, so each of them had mac & cheese once a day. I figured, "It's vacation, and DS6 is picky anyway, so let them have what they want." We were hard-pressed to find any fruits or veggies for them, so often brought it in as a snack or purchased a fruit plate as well.
We did not go to the princess meal in DCA, but that might be an option for you with DD6. Also, the Princess Fantasy Faire is a way to see lots of princesses at once, though I've heard mixed reviews that the lines are very long. Club Libby Lu (DTD) might be worth checking out if your DD would like having hair and make-up done like a princess.
Mid-day break at the hotel is probably a good idea to plan on. We didn't do this every day (sometimes DS2 fell asleep in the stroller, and then we just walked around or went someplace shady & quiet, like Court des Anges.) If we went to the hotel, DH and DS2 napped while DS6 and I hit the pool or did some other activity. One day DS2 didn't nap, and we went to Tom Sawyer Island to let them run around. (This will be closed for your trip, but you could go to the Redwood Creek Trail in DCA and do the same.)
I also wish we'd been able to see the Aladdin Musical. We heard such raves about it, and were so disappointed that it was closed during our trip. Do see this Broadway-quality production if you get the chance! Also the parades and fireworks are phenomenal!
Oh, and one more tip: My kids got a serious case of the 'gimmees' when they saw all the souvenirs and toys to buy. Before the trip I'd given each child some Disney Dollars to use as they wished, but I warned that once it was gone, no more. (What I really intended was to buy them something if WE felt like it.) It worked pretty well, except that DS6 found a really annoying toy that drives me nuts to this day! I tried to get DS to wait at least a day or two into the trip before blowing his money, but he just couldn't stand it, so we let him get it the next day. Be smarter than I was! Reserve the right of refusal, and check the toy out beforehand so you know if it's really obnoxious!
The park is so absolutely amazing, so I know you will have a fantastic time. Enjoy!