I like your wording and wish I had done something like you did. A few years ago we did a train theme party at a park with a real steam train that does 30 minute rides on weekends. The plan was party starts at 1pm, at 1:30pm everyone gets on the train. then cake and presents and at 2:50 we put pennies on the track and the 3:00 train will flatten them, (this was approved and suggested by the train employees and included at 10 minute child oriented safety lesson about trains and train tracks.) This would then end the party.
One of my sisters and her kids had said that they were going to be a little late and I didn't want them to miss the train trip so we switched things around pennies first, train ride second at 2pm. So the party starts, 3/4 hour passes we are handing out the tickets and conductor hats and they call to say they are in the parking lot. I told them great, park and run strait to the train I have their tickets. 10 minutes go by...still not there...5 minutes...and we still can't see them.We are the only ones on the train and the conductor very nicely volunteers to wait another 5 minutes. It seems they weren't in the parking lot they were at the entrance to the park which was a short drive from the parking lot. The train people were great and waited for them to come running on. The waiting put a damper on the kids excitement and dug into our cake and ice cream time.
I have decided that for parties with time sensitive activities I will be putting the details in the invite and then people will get to do the things they are on time for. If someone misses something I am not going to punish the rest of my guests because one guest can't make it on time.