My thoughts about whether you really need to get there early depend on your family's "style". Will you all be upset if you miss seeing things that are important to you? If you are there when it is hot, will standing in line for a long time cause your family (little ones) to melt down? Or, are you a "take it as it comes" kind of group? Reading the responses to your post, you can see there are "different strokes for different folks"!
My family has been a get up and go early family. Our first visit was when my DD was 7, and it was in June. She (and we

) had a long list of things we wanted to see, and we knew if we wanted to get them done, we had to go early. Plus, DH hates the hot, humid weather, so we wanted to be done at the park by lunch, and be back at the resort for pool/relax time in the afternoon. Also, my DD did not do well later at night, so our time to do rides etc. late in the evening was limited.
Now, she is a teenager, and getting up early is a challenge

Our last visit was during Christmas week, and we knew again that if we wanted to do anything at all we had to get there early. We did not always succeed this time, but that was ok, since we as a family had much lower expectations. Sleeping in was a trade-off that was worth it to us.
In short, if your expectations are to get tons of things done, and your family will be upset if you don't, then you may need to get up early on at least a few days. If you don't mind missing out on some things, then by all means, take it slow, and enjoy!
Either way, you will love your first trip!
