I'm not sure that's right. Googling around suggests that the average 5 year old knows somewhere between 3000 and 10000 words, and the average adult knows somewhere between 15000-20000 words. So I think saying a five year old has even 50% of an adult's total vocabulary is probably generous. Of course, if you meant that a five year old would understand about 80% of the words spoken by an adult over the course of a day, 80% sounds like a very conservative guess.

Not that it affects your point at all, I'm just thinking about words now. And while I'm on the subject, XKCD's "Thing Explainer," a book on how things work using only the 1000 most common words, is great fun.