Various fortune files attribute the quote to Robert Lucky. I would guess that it was misattributed to Alan Kay since quotes often get attached to famous people.
"In the future, you're going to get computers as prizes in breakfast cereals.
You'll throw them out because your house will be littered with them.
-- Robert Lucky"
"In the future, you're going to get computers as prizes in breakfast cereals. You'll throw them out because your house will be littered with them. -- Robert Lucky"
https://web.mit.edu/~mkgray/jik/sipbsrc/src/fortune/scene