Open collaborative design/Free and open-source computer-aided design
The availability of user-friendly open source CAD software will be essential to this field. There is no reason, with careful design, why this software shouldn't be intuitive enough for children to use easily. It could explain mechanical principles along the way if the user wished, and also be a place to store detailed contextual development notes, wiki style, to help others understand the workings and decisions made.
Another important area are truly open standards for associated file formats, and file converters to extract data from models created with proprietary CAD packages.
The virtual nature of the designs mean that far-flung people via the internet can easily work together on the same design, either working individually on various sub-assemblies of the whole or collaborating directly on the same part. The restrictions of having to finding people local to yourself with similar interests and desires becomes much less of an issue.