Kevin’s inflatable clown made it into his backpack earlier in the film.
Kevin only has his backpack and his dad’s bag when he gets on the plane to New York. He doesn’t have any of the clothes, shoes, or other things he packed in his suitcase for the trip to Miami (not that they would’ve done him much good in wintery New York).
But at the beginning of the film, Kevin’s mom gives him an inflatable clown that his grandma got him for Christmas, and viewers can see him shove it into his backpack.
Kevin uses the clown to play a trick on the hotel’s concierge (played by Tim Curry), which was perhaps a nod to Curry’s role as Pennywise the clown in the 1990 “It” miniseries.