The old Scooby Doo cartoons all had essentially the same plot: The gang and their dog try to solve a mystery that involves a ghost or some other supernatural manifestation. At the end, they solve the puzzle and pull off the mask of the unearthly bad guy, only to find that -- surprise! -- it is not anything supernatural at all, only Mr. Smithers (or the equivalent, a minor character in the first scene), who says, "I would have gotten away with it, if it weren't for you meddling kids!"
The Scooby Doo plot actually has, to use a dog metaphor, a distinguished pedigree. Part of the convention of the early Gothic novels, those scary tales of mysterious houses and dark and stormy nights that would mutate into the horror story, was to have a rational ending. However much they would raise the specter of the supernatural, at the end, the plots would always end with a rational explanation -- someone was dressing up, or staging a hoax, or misunderstanding an ordinary event. Only later in the 19th century, as Romanticism drove out the Age of Reason, would Gothic novels let the supernatural loose without reining it in.
Scooby-Doo 2: Monsters Unleashed (rated PG) follows the Scooby conventions but is a little fast and loose with the Gothic conventions, filling its time with comically implausible monsters in place of plot.
There is even something approaching a self-reflective theme, as the nerdy Thelma and the goofy Shaggy learn to unmask themselves, rather than trying to be someone they are not.
Scooby 2 is arguably better than the first Scooby movie. This one does not play to a knowing, older audience by, for example, implying that Shaggy is stoned. For better or worse, it is more in the spirit of the cartoons.
Very young children might be scared by some of the monsters. There is a little gross humor, of the sort that grade-school kids consider to be enormously funny. There are just a few mild innuendos that will go over the heads of most children.
Scooby-Doo shows that live-action movies can do basically everything that a cartoon can do, in the way of googly-eyed monsters, stepping off a cliff with legs spinning before falling, and other special effects. That is most of the substance of this movie, which will leave children entertained and their parents mildly amused.