I didn't really do sci fi as a tween, but based on those preferences and what my friends were into back when they were tweens, my top two would be the Discworld series and anything by Neil Gaiman. Of course I haven't actually read either, but I've heard good things about both.
Discworld was my shit, son. REALLY? AS IF IT WAS SOME KIND OF PINK PILL? NO. HUMANS NEED FANTASY TO BE HUMAN. TO BE THE PLACE WHERE THE FALLING ANGEL MEETS THE RISING APE. "Tooth fairies? Hogfathers? Little—" YES. AS PRACTICE. YOU HAVE TO START OUT LEARNING TO BELIEVE THE LITTLE LIES. "So we can believe the big ones?" YES. JUSTICE. MERCY. DUTY. THAT SORT OF THING. "They're not the same at all!" YOU THINK SO? THEN TAKE THE UNIVERSE AND GRIND IT DOWN TO THE FINEST POWDER AND SIEVE IT THROUGH THE FINEST SIEVE AND THEN SHOW ME ONE ATOM OF JUSTICE, ONE MOLECULE OF MERCY. AND YET—Death waved a hand. AND YET YOU ACT AS IF THERE IS SOME IDEAL ORDER IN THE WORLD, AS IF THERE IS SOME...SOME RIGHTNESS IN THE UNIVERSE BY WHICH IT MAY BE JUDGED. "Yes, but people have got to believe that, or what's the point—" MY POINT EXACTLY.All right," said Susan. "I'm not stupid. You're saying humans need... fantasies to make life bearable."