I can't see it as a second chance considering they're all trapped in a snow globe.
So many stories, so little time.

In short (for those who didn't watch the video explanation), it's because she didn't learn her lesson and didn't pass Krampus's test. Max learned his lesson about not losing hope, and even wanted to make a sacrifice to save his family (the theme of sacrifice is brought up a few times in the movie), so he passed and everyone got a second chance.

Check out this board.
http://www.imdb.com/board/13850590/board/thread/261021559
I responded to the original post there with a list of problems with the bad ending being true. The video link provided in the original post also has more evidence of the "happy" ending being the correct interpretation. Don't worry though. For the sinners out there, there is no happy ending in the film. Krampus dragged the truly naughty people down to the underworld that night.
Think about this, which interpretaion of the snow globe is correct. Krampus is forever watching them to see if they slip up and he needs to come back. OR Krampus is pleased with them, and wants to remind himself of the good deed he did with a snow globe so he can see his work. One is a
"
happy
"
ending, the other is a happy ending. You still can have a sense of dread with the
"
happy
"
ending.

I was assuming Max and his family were trapped on Christmas day in the globe though. Is that correct? Like will they just keep reliving the same day over and over? Also, I'm guessing Krampus had the grandma in a snowglobe also when he visited her for the first time? Which would mean she's kinda in two of Krampuses globes

Omi knew where Krampus took his victims, so she lied when she told how her story ended. She said she stayed hidden underneath her bed, so how would she know what happened to them? Let alone what were her chances of surviving a frigid winter with everyone gone for miles and miles around.
The best answer is that Omi lied because Max needed to repent his wish of his own volition and not just mimick what she did. It is just another instance that supports the "happy" ending.

So you turned it off before it ended?