Wicked! (All Six Books in One) by Paul Jennings and Morris Gleitzman
Something very weird is happening to Dawn and Rory. And it's not just their new step-family. Slurping slobberers want to suck their bones out. Strange steel sheep want to smash them to pieces. Giant frogs want to crunch them up. Cruel killer creepers want to squeeze the life out of them. Their parents can't help them. Dawn and Rory are on their own. It's wild. It's wacky. It's WICKED!