Geniuses are made, not born (my DC certainly is not one). Look up László Polgárs amazing experiment with his DDs which bear out my experience as a parent. Current thought seems to be that it takes about 10,000 hours practice for anyone to reach the top of a field, which is about 3 hours a day for a decade.
I would partially agree with Soooosie but would prefer to say, get them addicted to learning. The way I did this was to play fun games with my DC and every game involved learning. There is so many ways to do this, I could talk about it for hours. Every day from the moment they woke their was a new puzzle on the floor tempting them into solving it. Every car journey had memory games. Upon return from school an educational treasure hunt. Every weekend trips to the museum, science center, early learning center, the 100 acre wood, library. Every wall in the bedroom plastered in words, numbers, maps, science. Learning spellings became games of hopscotch. Dressing up and reenacting Greek myths when it rained. Every coffee morning I brought fun things to learn, while all the other mums just gave their kids some sparkly horse/car toys to play with. There are masses of normal pop culture kids books but look a bit deeper and there are kids books on quantum mechanics, black holes, chemistry, languages, history etc that are just as much, if not more, fun.
At bedtime we invented our own stories where we only advanced the plot by one tiny bit. Each character represented something eg a number and so he learn huge amounts of facts by making up stories/journeys one night at a time (memory maps). Pi to 100 decimal places, the periodic table, the bones/muscles of the body, flags, countries, capitals, planets etc etc.. Some thought he was a genius, others dismissed it as just an irrelevant memory feat. To us it was just stories we had made up, no different than any story he had ever read in a book.
Probably sounds like hot housing to many parents but it isn't, it is quality fun time together we both enjoyed. Even as a teenager we still do all those things but now its more of a challenge of equals. The results are an insatiable appetite for knowledge, just vociferous, he never watches TV, its inane and boring to him. I will admit his interests have tended to stray to the maths/science side because I found them more interesting as well but you cant have it all.
Y6 reading in Y2 is no big deal, once they go to secondary every child reads pretty well. DON'T do nothing or in the long run a bright kid will waste their potential.
Sorry for the rant but when I get talking about my DS...
To sum it all up make learning, fun and games all the same thing. Do a little every morning and night and you will open up a world of magic for ANY child. I wish I knew more parents in real life that felt the same.