Indeed, at the moment the BrillKids Library only allows you to upload Categories (*.cat) or Playlists (*.pl) for Little Reader files. We understand that in some cases it may help to have a way for everyone to share a full course they've created, but as course files tend to have very large file sizes, we are looking into the feasibility of having an option to upload course files at the Library.
In the meantime, you may continue uploading your categories. If you have a series of related categories (e.g., Animals 1 to 3), you may choose to export the series of categories as one file, and upload that one, provided that your file meets the maximum file size allowed for each upload (which is 20MB).
In the Add Download form, you may add some notes in the "Description" text box to if you'd like to explain your suggested way of using the file you're uploading.
A good alternative to courses are playlists - you can create a course-like lesson structure using playlists (it can be exactly as long as a full course, or just segments of a course) and upload those at the Library.
Sorry about this inconvenience, and I hope these alternative suggestions help!