What is best time for 9 year old 800m?

A 10 year old should shoot for a 3-4 minute 800m time.

The best 10 year old girl I've seen does a 2:47 so I guess 2:50's are good for a 10 year old girl.

It depends. The really good ones will do like 2:30. But that would be very hard. 2:45 would probably be a good aim for 10 yr old girls.