I found this brilliant teaching book for beginners whilst browsing through the sheet music at that treasure trove of a harp shop in Paris, Le Magasin de la Harpe.
Play the Harp with Charlie the Kitten by Isabelle Frouvelle
I've been using it for the past year for teaching my (very young) students and it's perfectly paced, with a good balance of music theory, little tunes and cartoons.
Each chapter section introduces one technical idea at a time in a progressive way, with a couple of preparatory exercises and then a little tune with fun titles like Charlie goes to School, Charlie goes for a bike ride, A Flea in Charlie's coat etc.
For when little minds start to wander, you can talk about the cartoons on the page as Charlie has lots of different adventures as well little 'spot the difference' cartoons. You can very easily just work through the book in page order, but after about 30 pages in you can start to incorporate into the lessons the duets as well as the more in depth theory and exercises at the back of the book.
For teaching regular weekly half hour lessons, you can cover technique as well as music theory in an engaging way without "boring" the student, or indeed them even knowing that they are doing so. It's an excellent book for teaching young students. Also an added bonus is that you can work from just the one book for some considerable period of time, so students only need to remember to bring one book with them to lessons!
Interestingly enough I've used this book teaching young girls as well as boys, and it has worked well with both genders. Sometimes harp books for young players can be very "girly" with 'twinkly' titles, but this book appeals to both boys and girls.
It comes in various European languages, so make sure you buy the book in the English language version. I haven't found it in music shops in England yet, but it's very quick and easy to order online from the shop in Paris here.