How a 4 year old and 5 year old hold a surprise secret

I neglected to mention that for Muddy's big surprise, the kids didn't know anything until Friday night. Once she left I told them that we were doing something fun this weekend (the story was that Addie and Ben wanted to take Muddy on the train ride for her birthday present) and I told Addie that we were having a surprise party for Muddy.
So Saturday morning I tell the kids I have to go pick up a surprise and I'll be back in a bit. Ben asked if he could come with me, and I said, "No, this surprise is so big, it fills up all of the seats in my car!" They were really intrigued. Then I said, "you'll be jumping up and down when you see the surprise." Addie looked at me, smiled and said, "I know what it is." "What?" "A moonbounce." Ben got excited and said, "Bye bye Spongebob!" Addie explained that my comment about jumping up and down gave it away. So then we had a discussion about what's wrong with their massive Spongebob Moonbounce, etc. I had to correct them to say that it was 100% not a moonbounce. I head off to the airport and pick up the New Hampshire contingent and bring them home, with Aunt Wendy too. When the kids come downstairs to see the surprise they see Wendy and are like "that's not the surprise" since they see her regularly. They turned the corner and were super psyched to see Uncle Jeremy, Aunt Amanda, LouLou and Bub. They said that was a great surprise. Then Onkteb arrived and Aunt Kim and Uncle Tom too.
We met the other guests as the railroad station and were allowed to have everyone on the caboose, waiting. Since Muddy was expecting us to be there, Brian, the kids and I waited at the main entrance for her and Guppy to arrive. While waiting we had to explain to the kids to keep it a surprise, until she saw everyone. I told them to act normal and not make a big deal at all. They agreed.
So Muddy and Guppy pull up to the station and as they're getting out of the car, Ben says "Our whole family is not here," but luckily Muddy was out of earshot. As she gets out of the car, Addie says to her, "everything is normal." I about died. I quickly commented on Addie's dress or something to smooth that over.
So that's how we managed to pull off the surprise of epic proportion.