You can enter at 4:00 p.m. and then do anything you want once you are in the park. Some people use that time to ride rides. Depending on what characters are there, some people will line up almost immediately once they get there for one of the most popular character meets to avoid spending an hour or more of party time in line for a character (for example for either
MVMCP or
MNSSHP 7 dwarfs and Jack (usually with Sally) often have huge lines and often start their meet and greets before 7-- although the 7 dwarfs have not shown up at MNSSHP this year, so they may not for MVMCP).
From the time DD was 2, we have always used a separate ticket and gone to a park on party days. DD was high energy, always wanted to be in the parks, and would stroller nap. We have done AK and then gone back to the resort and then to the party--- but we have found it a lot easier to do MK. MK typically has low crowds on party day until 4:00 p.m. too. For MVMCP, we sometimes just head to the park (not always at rope drop) and then stay through the party--- you can get your wrist bands at the party. For MNSSHP, we typically go back to the resort in the early afternoon, change into costumes, and then come back and go through the regular line (not party line) with our regular ticket and get wristbands at the station inside the park. We really enjoy MK on party days because it generally does have lower crowds in the morning and early afternoon.