We did the P&PP twice last year (March and August).
For the March one our boys were ages: 7, 5, turned 3 the day after the party (so he was free for the party yay!), and 11 months. The older 3 stayed up til the end and we had a blast!! We watched the first parade and of course the wonderful fireworks, and we did the treasure spots. We did some rides and they were walk on, even Pirates of the Caribbean which we thought would be busy. The only line we waited in was to meet Captain Jack, we waited about 45 minutes for him, and it was totally worth it. My older two got his autograph and he let all of the boys take a piece of his treasure (there were plastic coins). Our youngest fell asleep as it got late, so we just took turns carrying him and kept on going.
In August our boys were ages: 7.5, 5.5, 3, and 16 months. Again the older 3 stayed up til the very end. This time we decided to meet Pirate Mickey and Pirate Donald, which also had about a 45 minute line. We figured we'd sitck with ONE meet and greet since those lines are so long. And we did all the other stuff again. Youngest fell asleep towards the end again which was anticipated and fine.
But your youngest is 3 so she might make it til late like our 3 yr old did?! Whenever we do parties we plan a sleep in morning and easy morning the day of the party (and the morning after the party). That way we all have plenty of energy to stay up later. We don't go to any parks before the party. But we do go to the party before the official start time, usually around 4 or 5.
I will say that IMO doing the late parties like this and needing to have an easy day that day and allow for sleeping in the next day does take away some other touring time... If you need those early mornings in the parks to get things done, then maybe the party isn't a good idea. Know what I mean? But if you have gone before and will go again, and you are able to give up two mornings, then you would be fine. I imagine that others might do it differently than we do too. I suppose another way to do it would be to still get up that morning and hit a park early, but then go back to your room and all of you (or at least the kids) take a nap so you're ready for the long evening. My older 3 would never nap in the afternoon so that wouldn't work for us. Sleeping in does though. And everyone needs to sleep in the next day at least a little bit.
Good luck!
and Princess 
party, we were considering buying tickets. My girls will be 3 and 5 at the time we travel. Will they enjoy it at that age or is it a waste of $$$? Thanks in advance for your help!
Here's to hoping that they will actually extend the dates and we can all get tickets in May! 
We were planning Cindy's Royal Gala at the GF at 6pm. I might switch it up for another night, or try to make it earlier.
