What movie is this? Only a few details. Please help.?

I first saw this movie when I was kind of young (about the late 90's I'd guess) so the details are kind of vague. I remember that these kids (I think there's three) have to go rescue their parents from some weird place like a town but there's weird stuff there (like I remember the oldest kid, who's a boy, bumping into this pinkish thing and the whole place kind of roars and he says something like "it must be a nerve") The main thing I remember is in the end they get their parents and drive away. It then shows them having a birthday party or something and when they cut the cake green slime starts coming out of it, then the kids look at their parents and there's green slime coming out of their mouths meaning that the people they rescued were not their parents. I know it's vague, but if anyone knows it that'd be awesome.