Continuity mistake: When Lavagirl's hair is on fire due to her anger, Sharkboy splashes her hair to put it out. Between shots her wet hair instantly dries and becomes fluffy and neat. (00:57:30)
Continuity mistake: When Max is about to climb up the tower of ice to retrieve the Crystal heart, Sharkboy hands him his left glove. But in the shot of Sharkboy falling down the ice tower, both his gloves are back on! (00:59:25)
Continuity mistake: At the end, where Marissa has just defeated Mr. Electric, Linus runs out of the room and into the snow. It's not the actor but instead a double running out of the room. In the next shot, he pushes past Max to get out of the classroom again.
Continuity mistake: When Minus has the heroes captured and hanging down from wires, there is a shot where he is threatening Max and Sharkboy is nowhere to be seen. There is no wire either, so it's not like he's out of sight.
Answer: Lava Girl is always hot to the touch. She can control when she throws magma, but the only way she can have any control over her actual body temperature is to control her emotions, which would naturally be difficult for someone her age. Even then, at her coolest possible temperature, her body temperature would still be very high. There are continuity issues with her body temperature throughout the film, ie, being able to sit on ice for extended periods of time without melting it, but the idea is she always has a very high body temperature.