D7000 with whcih prime lens

Finally decided to upgrade from my D80 to a D7000

But I want a good prime lens to go with it...

Most of the stuff I shoot is of my kids so my priority is that it has fast AF. I'm prepared to sacrifice some quality to ensure the AF is as fast as possible.

Ideally I want a 35mm - the crop on a 50mm would probably make it too long for what I want.

I've always had Nikon lenses until now and would only deviate from that if there is a really strong alternative.

Price range: Maximum $500.

Any help/advice gratefully received.

Many thanks.