80% of browser functionality is written in JavaScript, very similarly to how Mozilla's XUL is done, greatly reducing our dependency on any client platform.
But we are Windows-only at the moment with Mac OS X in the very near future. We are also debating either we should move to Mozilla platform, but it potentially will make a fairly fat download... Currently our browser is 400kb binary with zero dependencies.
I'd move over to FF base and you have all OS' covered. If you have to install a binary, people don't care if it's 400kb or 2Mb. Size, under 10Mb is irrelevant to most of the people. Usability and compatibility is what's important.
Could you put up an interactive demo with said 80% of the functionality, just running in your browser? It wouldn't have to do all the "shell sandboxing" that the full client does; it's for the parents, not the kids.
It would be great to see some screen shots of the browser on the web.
Does it run in any OS?