Kai was 3 years old in 2003 I know it's may not came out in 2003 but go back to the season 7 you will see that Kai was looked as 3 years old and Nya would be 1/2 or 1 years old or I am just confused.
Then 2 years later, Nya is 3 year old and Kai is 5 years old but then axronix token there parents away from Nya and Kai.
Nya and Kai has successfully survived their Life without there parents away for 9 years, they learn by there self and training there skills without being and ninja then in 2011(9 years I said) they found Sensi Wu and trained as ninja.
They only dished everything was normal they can see there family and friends to help.
Meeting Wu (Master of Creation)
Wu found Kai and Nya but Nya didn't come so she just stay in the blacksmith, but Kai was training hard to be able to pass his test to become a Ninja though surprisingly Kai failed in 3 times so he tries again then he passed but Nya trained when she was 15(season 6)as she keeps failing she couldn't control properly. But now she noticed that she was doing wrong so Kai was master of fire and Nya is master of water.