Totally Wicked! by Morris Gleitzman
Dawn's dad and Rory's mum get together and the children are not pleased, they just don't like each other. The threat of the Slobberers, horrific huge worm-like creatures with slavering tongues that suck the insides out of creatures, makes them almost forget their rivalry. This is only the beginning of the bizarre and awful adventures the family must go through before any happy ending is in sight.