My reasoning behind why Aang and Katara had 3 children with different bending or whatever: one with waterbending, one with airbending, and one with no bending... Aang was an airbender, Katara was a waterbender, and Sokka who by the way is related to Katara was not a bender. Sokka always thought outside the box and from the few scenes we saw of Bumi, he definitely seems the same way. Tenzin is laid back but serious when he needs to be much like Aang (look at adult Aang if you want a comparison). Kya, we have no clue but I'm highly betting she's quite a bit like Katara, nice, caring, etc.
The creators did that on purpose, there is no doubt in my mind that their children resemble the three original characters that we first met in the first episode of Avatar... The one that started it all.