Hello Amy.
We had the same needs back in July as we have some classrooms with thin clients and although we don't just offer the oVirt service we wanted something simple so users don't need to open a browser and just handle their machines in a simple way, so I found this project [1] that works quite fine for what we need. Manage virtual machine's power and start a viewer. Period. No more funcionalities (not sure if you need something more sophisticated...).
The project has been stopped for a time but it seems that the maintainer is implementing new capabilities and fixing some things lately.
As a side note, you didn't mention if your thin clients are linux or windows based but I believe this is for linux only.
HTH,
James