How did Goten and Trunks become Super Saiyan so easily?
According to this Yahoo answer:

Because by the time they were born Goku and Vegeta had already been Super Saiyan and with the higher power level thus when trunks and goten where born their levels were also higher.
Think about it this way Vegeta (Fut Trunks timeline) was weaker at SSj Level so Future Trunks took longer to become ssj (i think he was 13) while Vegeta (regular DBz timeline) had trained harder by knowing the androids arrival 3 yrs prior to chibi trunks being born thus more powerful ssj1 vegeta. And Goten being born around some time near end of the Cell Games saga Goku had became already full power ssj.
Thus as children their power levels were already exponentially higher than the latter of gohan who was born back when goku was weaker in power level.

Basically Chi-Chi gave birth to a Demigod.