We usually try to get an ADR at CP around 4:30 or 5:00. You can get in the park with just your party ticket and be finished eating by the time the party starts!
Our adr for the last 2 years has always been around 4:30 or 5:00 too. After a big meal the teens are well fed and ready to go for the night. Make an ADR as soon as your plans are finalized because they go pretty quickly. I am bad. I usually wait to see who the CP processional narrarator is lst at epcot, then I decide what MVMCP to go to.