Question: Does Sharkboy seem to have a problem with Max? I dunno if that's just a vibe I got from him or not.
Answer: According to Taylor Lautner, the actor who played Sharkboy, he approached the role as if Sharkboy was jealous of Max because Sharkboy has a crush on Lavagirl and she pays Max lots of attention.
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.