At a theme park, planning is a parent's best friend. Mixing greasy food and dizzying rides can ruin a good day. No one wants to spin off the teacup ride and head straight to the nearest trash can. Plan to visit certain rides well before or well after eating.
You should also have a realistic expectation about how much excitement your children can handle. Some theme parks offer more activities than you can reasonably pack into one day. Evaluate the staying power of your party. Young children or elderly family members simply won't be able to hold out as long as adolescents or teens; don't exhaust them. Make it clear to everyone that your time at the park must be enjoyable to everyone, and that you'll yield to the health and safety needs of the weakest member.