What is best than a good RPG ? A bundle of them ! If you do not believe me : check 24XX srd, free of charge. Then, imagine the same installation for most flavors of scifi settings you can imagine. Then imagine them bundled all together, for a bargain price. Wait, no need to imagine, it is right HERE in front of you.
Shake and wipe them as you require to have YOUR scifi seeting, from Star Wars oriented to John Carter of Mars, from Eclipse Phase to Cyberpunk. Or a freaking mix of all ! I played with four kids on ten sessions small campaign, on a Star Wars like settings, with a pinch of W40k. It worked pretty well and the price paid is worth it !
The only kinds of player / GM it may rebuke are the ones who do not like the "player facing" dice handling approach, or the ones who appreciate more simulationnist RPG.