After the burglars have been taken out of commission and arrested, Kevin's left with an empty home. He apparently does quite a bit of cleanup—since his mom doesn't slip on the stairs when she later arrives. (Maybe Kevin even salted them to eliminate the ice trap he'd set for the burglars? We don't know for sure.)
When he wakes up the next day, he's disappointed to discover that Santa hasn't given him his family back. After running the gauntlet with those burglars, it seemed like he earned his way out of his solitude. That, of course, is the moment when his mother arrives—calling out to him as she enters the house. The polka band has safely escorted her back to Chicago, though not without torturing her ears (we can assume).
So, someone (Santa or whoever) granted Kevin's wish to have his family back—though, by defeating the robbers, he's poetically earned it. He's proven himself worthy of having a happy ending.
The rest of his family walks in the door, having taken the flight that Kevin's mom didn't want to wait for. Everyone's impressed by how well Kevin managed to fend for himself. Thanks to his experiences with the burglars—but also more mundane tasks, like shopping for groceries and doing laundry—he's become a fully competent kid, capable of surviving on his own.
He tells them, in addition to doing these household chores he "just hung around." They all seem surprised, and Buzz can't believe it, saying, "He went shopping? He doesn't know how to tie his shoe!"
Kevin's undergone what all central characters are supposed to undergo—change. He's brought back the heroic elixir from his struggles and can now be a better-equipped member of the household. We can assume that his newfound abilities will carry on into the future.
After their reunion, Kevin's dad finds Harry's gold tooth lying on the floor—it was apparently knocked out at some point during the mayhem and Harry couldn't find it.
Since Kevin hides the truth about the burglars from his parents—which, in and of itself is kind of suspicious—this is the only remaining piece of evidence. That is, unless old Marley spills the beans and tells Kevin's parents what happened—which he probably should do, considering.
In the movie's final moment, Kevin looks out the window and sees Marley hugging his granddaughter and reuniting with his formerly estranged son. Thanks to Kevin's help—encouraging Marley to confront his fears the same way Kevin confronted his fear of the basement—he's been able to revitalize his own life. Kevin's courage has radiated outwards, and is a boon to other people as well as himself.
This is definitely a classic Hollywood ending—and why shouldn't it be? Imagine if the burglars actually bit all of Kevin's fingers off, like they threatened to, and then threw his corpse in a dumpster. Suddenly the movie wouldn't be funny anymore—it would be really sick.
So, you have to stick with the comic, heartwarming tone. Bad guys get their just deserts, good guys win, and everyone gets home in time for Christmas. Perfect.
At the very end, we hear Buzz yelling, "Kevin! What did you do to my room?" Kevin looks surprised and goes to run away. But since we now know how crafty and resourceful—indeed, devious—Kevin can be, we imagine Buzz won't pose much of a problem.