Don't tell anyone, by there's a special temporary Hero Kids bundle up on DriveThruRPG. This bundle includes all the PDFs: Hero Kids core rules and introductory adventure, six normal adventures, and the one premium adventure.
The price is a rock bottom $9.99, which is a massive 67% discount over the full price for all of these!
Grab it here (while it lasts):
Hero Kids - Uber-Discount Bundle!