I also think D90 is a great choice ( I am using the D90). 
The good thing about D90 with regards to portraits photo is its higher ISO. It is about one and a half stop better than the D80. This means that the shutter can go much faster. Kids do run around quite often.
For landscape photo, you will need a good tripod.

The good thing about D90 with regards to portraits photo is its higher ISO. It is about one and a half stop better than the D80. This means that the shutter can go much faster. Kids do run around quite often.
For landscape photo, you will need a good tripod.