Maybe it's a self-fulfilling prophecy, that you're often interested in what you're good at, and good at what you're interested in. The inverse of these is more or less true as well.
The point is that trying things and seeing what "sticks" can probably lead to what they're both good and interested in. At least that's the hope.
My goal with my kids (who knows if I'll succeed), which is sort of what my parents seemed to do with us, is to encourage things we liked (not like, video games, but constructive things like music, sports, science).
Prior to university, hopefully they pick up some interests in things which will translate to a useful University degree. By that point, I would assume you would know if it's a waste to spend money on it or not... I suppose we have to save either way though right? ;)
Re: sports: you look at some of the great athletes, and these are always going to people who started out as kids who just loved doing what they did. Tiger woods was playing when he was 2. Mike trout would sleep in his baseball jersey when he was 5 years old the day before and after a game. That sort of thing. Of course, those guys have super human skill on top of the practice and desire, so there's that part of the equation that needs to be there as well.
The worst are the parents who keep pushing their kids to play a sport even though the kid stinks at it and is having no fun.