The app is free to install and sells things once you are inside. On a child's device this is the first setting to find — require a password for every single purchase.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
free entry to the fair, paid rides inside.
For example
The game was free. Your child taps a few times and a cartful of coins is bought, because nothing asked for a password.
This is one card out of 275.
The hundred words you meet most are free — the whole set, animated, in one file that works with no internet.
Volume 1 has all 275 in fifteen chapters, with Staying Safe the largest chapter on purpose.