A good laugh
The campsite owner would observe the campers first to see if they would be able to appreciate a prank or not. When he saw the Millers, he thought that this was a family that loved to have fun and it looked like they could handle a good prank.
Source: SmartWorldMag
While Jason and Macy were still in shock, their kids were laughing. The idea that their parents had been pranked made them happy. Soon, their laughing was getting contagious and everyone ended up laughing too.