Alan Kay's work is very much about children, but Engelbart's project was about (qualified) grownups and future civilization. (Children at the age of about 10-12 were especially interesting to Alan Kay, because they're at in a transitional state from visual dominance to symbolic dominance in thinking, following the work of Piaget and in the updated version by Jerome Brunner.) Regarding, what is discoverable, the civilizing aspect may be well in what is shown and in what way. E.g., Licklider's examples are actually conversations about constraints, without verbalising constraints specifically.

Edit: One notable "relict" of Engelbart's project is the outline view in MS Word (which, out of context, may not appear to be that remarkable, while it was much about how texts should be organized and understood).