I was unsure about the "for kids" but went with it anyway, on the grounds that I get charged per click on the ad so I really want to make sure that the people who click on it aren't just doing it because they can't see what the writing on the book says...
The result, after 5 days, is that the ad has been seen by 11,973 people, an average of 4.7 times each. It has been clicked on precisely 12 times. I thought these would be entirely by accident, but no: remarkably, those 12 clicks have led to 2 sales of the book.
However, to get those 12 clicks, it cost £2.93 in advertising. If I didn't have £30 of free advertising, I'd basically be paying £1.50 to see each copy of the book. This is just over twice the price of the book itself, and five times as much as I get in royalties from it. It's not, therefore, economical.
Mind you, if I were to raise the price of the book six-fold and the purchasers are price-insensitive, that would work.