Hi jeankeri,
We didn't get to the party until 6:15 because first we went to the character meal at the GF, we went in at 4:40. Mom likes the music and atmosphere of the GF and after the meal, we let her sit and enjoy, they had musicians. DH and I walked around, it was a beautiful night.
There was not a line to get into the park. One of the things that mom wanted to do was the Carousel of Progress, and that was good because they had a line down the middle of main street and we had to go to the right before the main intersection. This area, walking into Tomorrowland was crowded, but we walked right onto CoP. We did some trick-or-treating and waited about 10 minutes to have our picture taken where AE had been.
As we walked to PhilharMagic we noted that we could just walk on to Winnie the Pooh, but didn't, and there was no line for Snow White. PM was Great! The only reason we had the wait for PP was because we went right after the PM show, and so did lots of people. As we exited PP, there was NO line at all to enter. We walked into the room at HM, but that's because we were willing to skip the parade which was going to begin shortly. We also did the river boat to hear the story telling.
I think the party is a great way to begin the Disney Vacation, your family will love the parade and fireworks, and because of the smaller crowd good viewing places are easier to find.
The letter that came with the ticket mentions getting PS at the Liberty Tree Tavern, perhaps that's something to do for the wait, and it would also get you to the other side of the park before 7. We got to the MVMCP last year before 7 also but not early enough that I can tell you about crowds much before the party begins.