Randall Munroe (of XKCD fame) wrote "Thing Explainer", a book that attempts to explain things using only the most popular 1,000 words in the English language: https://blog.xkcd.com/2015/05/13/new-book-thing-explainer/

I also remember having a book similar to this around that age (maybe a couple of years older): https://www.dk.com/us/9781465414175-knowledge-encyclopedia/

The nice thing about the latter one is that the pictures will be enthralling now, and as your child grows they'll be able to understand more and more of the book :)