The floating of kids were not in the book. The director told GameSpot:
"That was, you know, sort of metaphoric in the book. In my mind I suddenly pictured these guys literally floating in the air around that grotesque pile of toys, and it seemed like a great idea".
"[In the book] it's some sort of symbolic depiction of dying, and floating is like, you know, 'Your soul will come into my inter-dimensional realm,'" he continued. "But I didn't want to introduce the macroverse and the inter-dimensional cosmic element into the story, because It, for me, was so much about the Losers and their journey, and I wanted to keep the story as pure and as focused on that point of view as possible."
But there is no explanation as to how Pennywise does the floating. Maybe it will be explained in the sequel.
From reddit, by rdhight:
It exposes them to the Deadlights. We don't know how the process works physically, but that's what we see happen. It opens Its mouth and shows Beverly the Deadlights; her eyes cloud over and she floats. While she floats, she has a vision of the future.
A person who lives in this world is forcibly connected to something from outside it. As a result, she goes partially immune to this world's laws.
I'm guessing the kids that were floating weren't afraid of IT, as Billie says to Pennywise as he was about to fall down his hibernation well,
"That's why you didn't kill Beverly, cause she wasn't afraid, and we aren't either, not anymore, now you're the one who's afraid, cause you're gonna starve".
This just represents the fact that, if the kids that Pennywise captures are no longer afraid he makes them float.