I stumbled into this place and it looks like someone may be able to help me with this...

I've been after a movie title for something like the last ten or fifteen years, and just lately it's started bugging me afresh. I must have seen it when I was super small, and only remember fragments of the ending. Anyway, here's what I've got:

These (seemingly) small kids are in this huge black room, and the floor is cracking up so that they can only stand on spaces jutting up, and they're trying to find a pathway out of there. There's some old man in a suit spotlighted, and apparently he's evil and trying to trick them. Also, there are tall, demonic looking figures/creatures trying to grab the kids. Jump to my next piece of memory, the end of the movie: some of the kids are back home in "the real world", and suddenly there's a huge explosion outside. Apparently something from that "other place" has bombed or landed and destroyed half their house, only I think they're the only ones who realize that. Neighbors and whatnot are shouting and trying to figure out what just happened, and one child finds his microwave on the lawn, opens it and finds a special rock or giant stone or something that apparently had massive significance. Besides that, all I remember is that the ending credits had a map in the background.

I know that all sounds insane and like a dream, but I swear it has to be out there somewhere. I haven't really thought of it in several years, but a friend and I were talking about kids fantasy movies in the 80's, and it triggered these memories and now I'm on a search rampage.

Help! I'd be forever grateful if someone else knew what I'm talking about. Thanks again. -Vanessa