My kids have been all different ages for our trips. The youngest we have had was 11 months old. We are "comando" park people thought. We go to rope drop, rarely (as in once) have we ever left for a break during the day. The kids have always napped in the stroller, so we usually see the fireworks and parades at night. But we plan our days fairly well, as in, when we eat, and do shows for a break without leaving. But my kids have never been very scheduled when it comes to bedtimes, our schools don't start until 9am, so they are used to being up later, and they will get up at WDW better than anywhere in the world!
That being said, if this is your first trip, I would try to do at the fireworks and parade at MK, and Fantasmic (our favorite). DH loves Illuninations, but for me it's just OK, if you have a Star Wars fans, then the fireworks at DHS might be a must too.
Our shortest trip was a quick 2 days on our way to Clearwater, but other than that for our normal trips we are usually there for 10 days, (arrival day, 8 park days, and departure day- usually 7 day park hopper water parks and more tickets)