To answer the OP's question, yes, definitely expect crowds especially after all the locals get off work and make their way to the parks. It was jam packed when we went on leap day! We arrived around 6:00 or 7:00 and DLR was at capacity so they weren't letting anyone in once we got in line. We waited about 15 minutes before they reopened the turnstiles. However, we expected crowds and figured lines for rides would be long. We took our time and had an absolute blast. Had breakfast at Cafe Orleans at 3:00 a.m. and watched the sunrise which we've never done at any Disney Park. If you go in with reasonable expectations you can still have a good time.
ETA: Oh, one more thing. The atmosphere for the event was more "adult" than usual so I wouldn't recommend taking little ones past a certain hour. Besides, they'd be too tired to enjoy themselves. I saw a lot of parents with little ones even during wee hours, not cool.