I think the cartoons I watched as a child definitely started off my love of SFF. It was all about anime for me then;
Battle of the Planets, Astroboy, Robotech, Starblazers and even
Speed Racer. Two things really drew me to these shows, firstly the animation, as soon as I saw shows like these,
Simba the Lion and others I loved the realism of the artwork. I never liked cartoon characters out of proportion throwing acme anvils at each other. These shows looked great and acted more real, the physics worked and the actions were at least kind of believable, well for a 6 or 7 year old they were close enough. I suppose what I mean is if something happened they usually gave at least some sort of explaination as to how.
The other big thing was that there was an ongoing storyline, in some of them quite detailed. Characters were introduced, were even killed occassionaly, people changed and season three things weren't the same as season one (to varying degrees with the shows). That's something that can be hard to find in alot of 'mainstream' shows today.
Still they were my 'gems' in a whole lot of rubbish and I think there's still some of that today, the real stand out for me would be the
Avatar cartoon, I dont watch it but have seen enough episodes to think I would have been a big fan as a a kid.
There's not alot for the older children/teenager age group but then again was there ever? Im sure there were some, I liked the
Tripods BBC show but I dont remember there being a huge choice,
Star Trek will probably always be around in an updated form and
Buffy the Vampire Slayer had a pretty good run.
I also wonder if gaming and particuarlly online gaming isn't filling a bigger role now for this age group as far as developing an interest in SFF?
Things go in cycles, super heroes are still very big and after a bit of a boost due to the LOTR movies I think TV's a bit hesitant on Fantasy right now. I think there's still enough SFF out there to interest kids, some of it even watchable but as for increasing the market at the expense of reality tv that might be pushing it

.