"There is no mistaking a real book when one meets it. It is like falling in love, and like that colossal adventure it is an experience of great social import. Even as the tranced swain, the booklover yearns to tell others of his bliss. He writes letters
about it, adds it to the postscript of all manner of communications, intrudes it into telephone messages, and insists on his friends writing down the title of the find. Like the simple-hearted betrothed, once certain of his conquest, “I want you to love her,
too!” It is a jealous passion also. He feels a little indignant if he finds that any one else has discovered the book, too."
If they're actual leaflets they shouldn't be registered at all, but there's nothing wrong with registering those very slim softcover children's books. I'd leave them in the same places I leave anything else - playgrounds are a good option, waiting rooms at places where someone might bring children (dentists, pediatricians, hair salons...) or even where they wouldn't (the grownups might like to take the books home for their kids), and of course OBCZs, Little Free Libraries, or other book-swap shelves. If they're really lightweight I wouldn't leave them outside on a windy day unless I could put a rock on top - or could hang them from a tree in a plastic bag - but otherwise I wouldn't see any limits. Heck, it's the books with extreme adult matter that cause more "where can I release this" headaches!
My local supermarket used to have child-sized shopping carts, and another had those carts with a sort of "car" on the front where the kids would sit. I would always have a book or two to leave in one of those places.