Continuity mistake: Right at the beginning, the kid's hand keeps swapping from being on the middle of the paper to on the upper part, between shots.
Factual error: In the first scene, the math problem is wrong. It says "Grave buys a bag of 240 jellybeans. There are 35 yellow ones, 52 red ones, 63 green ones, 26 white ones, 41 blue ones, and 40 black ones." That is 257 jellybeans not 240. Don't ask why I stopped and did that math.
Continuity mistake: At the end of the scene where Speed and crew are ambushed by Cruncher Block in the mountains, Speed is pointing a gun at Cruncher Block. When Spritle says "Take him away, boys", we see Speed in the background lowering his gun, but, in the next shot, the gun is raised again.
Revealing mistake: Throughout the racing shots of the movie, the close up shots of the fans in the stands show the same people used multiple times as evident by them wearing the same clothes, poses, etc.
Answer: They're a close-knit team. Sparky's a close family friend as well as an employee. It's rather like Race Bannon living with the the Quests as part of the family, even though he is employed by Dr. Quest.
Phixius