The '06 Pro Kids Go does come with a center fin box.
It's got some performance, but depending on your daughter's weight, and her ability to handle larger rigs, it may be awhile before she can fully tap into the performnace of the PK Go.
One of the earlier Starsurfers might be better.
What size rig (s) will she be using......?
This can be a very critical factor in planing performance and her ability to stay upwind.
Here's a link to the data on the 2006 archive:
Hope this helps,