Well, first of all, I think that their kids are definately going to be airbenders because of two reasons:
1. Aang needs a lineage of airbenders because the airbending tradition needs to carry on after he dies; what's the point of having no airbenders when the avatar is reincarnated? (the avatar won't be able to learn airbending, and the cycle might be broken).
2. On Nick . com, in the airbender history, it states that all people born from the air nomad tribe are airbenders. ALL of the air nomads are benders. This, to me, makes me believe that airbending is a dominant trait.
And really, Aang is a member of all four elements. Remember when he was working with that guru, about how to open the chakaras and control his avatar state? he said "You are the avatar, and that means that you are a firebender." Now, whether it's genetic, probably not--but who's to say genetics make the bender? If bending is genetic, how could Aang take Ozai's firebending away? by changing his genetics?