Don't let them get the peanut butter and jelly and marshmellow sandwich if you come across it anywhere. I had it when I went as around their age and it was gross.
I got to go a second time when I was 15 or 16 (yea, I got lucky my grandparents had never been and had always wanted to go, so we got Disneyland twice). From that, I will say, get the fast pass if you can afford it and ride roller coasters earlier if you can/are going to. This would be more for your entertainment than your kids since they're young, but your 5 year old is probably old enough to get a little pissed from waiting in lines at times, so it might be worth it for him, too. Though the cost of it vs reward is really going to be based on factors outside of predictability; think of it like insurance, if you want. If you're going in summer, probably more likely that it would come in handy.
Lastly, set an established rule for getting lost. My parents' rule was to stay in the same place and don't move if you find yourself not with them. And yea, the fuckers walked off without me one time when they thought I was tagging along (when I was 5, not 15; I wasn't an invalid then :O). I just stood where I was and they got me a couple of minutes later.