Regardless of how you try to justify it, Sharkboy and Lavagirl in 3D is a bad movie. Spy Kids 3D (the obvious comparison) was the weakest of that series and yet it still towers over Sharkboy in terms of plot and well... sense of fun. Even worse is that Sharkboy commits that cardinal sin of children's movies: actually talking down to its audience.
And yes, I know it's a kid's movie and all, and I shouldn't have high expectations but none of the kids in the cinema with me seemed particular engaged. In fact, most seemed bored by the story, although completely wowed by the 3D effect. The standout moment came for me when Lavagirl died (In children's movies, all heroes are contractually obliged to die or appear to die) and some kid behind me shouted out "Deadgirl".