In a real sense, your market is not the kids who'll be using the software, it's the non-profit organizations (and possibly also government organizations) that can promote the software and give it the infrastructure to get it to the end-users.

I think if you find a few international assistive-technology email mailing lists to post to, and get the attention of a few local or regional non-profits who are willing to look into software for handicapped kids, they will do a lot ot get your software to its intended end-users. If you can find good nonprofits they will do much better marketting than commercial vendors, since they will be talking to people who probably trust them and may have even provided free services to them before. The software's merits will mean more to the end-users than any perceived problem from its free cost.

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-DA