I've not actually done one of the portraits (a couple DISers have). But I did ask about it.
You can make an appt. You probably wouldn't need to schedule too far in advance. It looked like the artist had a lot of down time where she was available.
On drawing youngsters, I think she had an age cutoff where she would do them "live". She takes 30 - 45 minutes of still time, and on kids under 4 (???) she'll need a picture to draw from (this is what pulled me away from doing it).
Cost was around $80 for the portrait. Seems like she would do something different to it (matte it???) for an extra charge around $120. Also, the more the merrier (and the more expensive) - more than person increases the charge.
The couple of DISers that did it spoke well of it. It was a tradition we were going to start with DD, if not for the drawing from a picture. It kind of took away capturing the moment right then.