B-21 HOW CAN THE KIDS BECOME SUPER SAIYAJIN SO EASILY?
(This answer is from Alexander Chen)
Nobody really knows how the kids were able to achieve SSJ with apparently so little ease. All we know is that they did. This being the case, there have been various theories thrown around. I can't say which is right or wrong, but I will include the two which in my opinion seem most plausible. These may be entirely wrong, of course, but as yet, I have found no better theories:
One theory suggests that it is due to genetics. We know that hybrids (i.e. crosses between different species ) often produce stronger strains when done with plants. We can extend the argument and propose that it has had a similar effect here, in other words, a half-human and half-saiyajin hybrid is much stronger than a pure human or a pure saiyajin. Obviously, this is a lot more complicated than plants, as strength and power in warriors are increased by training. However, this may explain why the kids were able to attain a higher power level much earlier than the other warriors were able to. Remember that Gohan also attained a much higher level than Gokuu did when he was Gohan's age.
Another theory may be that once a saiyajin has managed to become a SSJ, their genes also change in some way and they can then pass these on. Both Goku and Vegita had become SSJ before Goten and Trunks were born, whereas Goku had still not attained this level when Gohan was born. This may explain why Goten and Trunks were able to become SSJ much more easily than Goku, Vegita and Gohan (Bardock and King Vegita, the respective fathers of Goku and Vegita, were not SSJ).
These are just two of the many theories that have been proposed. In my opinion they are the most plausible two. The real answer to the question "How can the kids become Super Saiyajin so easily?" is that nobody really knows! Apart from, perhaps, Akira Toriyama himself! (There may even be no rational reason for it--it was just something that he decided on! =) )