Once school gets started, KidzTalk will be a lot more busy. KidzNet is more for keeping in touch with your own circle of friends, but you should also use KidzTalk to post to a wider audience, like stories, or have a general question for homework, advice, etc. They both serve a useful function.
KidzTalk allows posts without accounts, so for people that don't have them, that is another difference.
