Trying for "whiz Kid" aspiration, but how do i "read with an adult for 2 hours"?

The action is available on children's books. Make sure the book you're selecting is a children's book. Lord of the Swings and My Pizzi-cat-Polka (title approximate) are two that should be in the bookshelf by default.

Edit for clarity:
With an adult sim active, open the bookshelf, and click on a children's book (or use one in the adult's inventory). Choose "Read with..." and then select the child sim.