5 Causes We Can Be Heroes is the proper sequel to Sharkboy and Lava Lady
That said it also has Sharkboy and Lavagirl in it, as well as their daughter, Guppy!
Taylor Dooley, who played Lavagirl in the first movie, returns and she's all grown up! And while the other Taylor chose to sit this one, his replacement, played by JJ Dashnaw, is fine even though he doesn't speak. But the film is not about adults (more on that soon). It's all about the kids. And We Can Be Heroes is a natural progression in the story, as Sharkboy and Lavagirl had a kid named Guppy together!
Guppy isn't the main character in this film, but she is an important part of the team. She has the strength and teeth of her father and can chew metal bars. She can also get into a shark frenzy, much like her dad (wish she had lava powers too, like her mom, but oh well). So I am basically saying: if you enjoyed the first movie, you will be rewarded for watching the second to see the adventures of Sharkboy and Lavagirl to follow!