little help, please?

[phoenixdagger.livejournal.com]

So I'm visiting my cousin in a week or so and -being both broke and crafty- I want to knit a hat for her little daughter.

BUT! Since I don't have a lot of contact with toddlers and because running up to children in the mall and measuring the circumferences of random children is generally frowned upon, I was wondering if I could get some advice here.

What sort of measurements would I be looking at for a knit hat for a two-year-old? I'm using sport weight yarn on size nine (US) needles.

Thanks, all!