I can't tell you how frustrated I was with this episode!!
.........rant
Its only the second episode, so much can change from here so don't give up.
Aang is not a perfect person, he was just the avatar which means he was not perfect. You only knew of his character in his teen years as a playful kid, you don't know how he was as an adult or parent(People mature and change). Him favoring Tenzin over his other kids makes TOTAL sense, like how did you not see that coming. Tenzin was the only other airbender alive with Aang after so many years Aang could finally connect with someone else because of airbending, of course he is going to favor him more. Doesn't mean he doesn't love his other children. He had to teach Tenzin about all the air temples and stuff, what are Kya and Bumi going to need that for?
As for Kya and Bumi, can you blame them. If Aang didn't pay as much attention to them as he should have of course they have the right to be mad. Just because he is dead doesn't mean you cant speak of his mistakes.