There is no guarantee...even for on-site guests. We got to EPCOT this past NYE about 10:00 am and it was starting to get crowded. We chose EPCOT because it is the largest park. We managed...my DDs were 6 & 10...by taking a lot of breaks, just sitting, having a snack and people watching. We left WS around 10. There were a lot of people, some drinking, and a whole lot of noise makers...very loud. My youngest DD kept getting hit in the head as we walked. Her head seemed to be right at elbow level. Once in FW, we walked on to several shows and enjoyed them while resting. I believe they did close the park that day in the late morning. We actually had a good time, but it will be a while before we go again!
Michele