I wrote The Dragon and the Thief and The Land of Gold for my own children. We enjoyed it very much, and they've had excellent reviews, but both are now out of print. I enjoyed writing them perhaps more than I've enjoyed writing anything else I've done--probably because I wrote them as bedtime stories and had an immediate and enthusiastic audience. The monsters were all invented by my eldest son. My brother Alan did a screenplay which is excellent and won an award but never got made into a film, if there are any producers out there reading this . . .
To my great delight, these two books have just been reissued as a single volume by blisslife books. See http://www.blisslifebooks.com/index.php
Beyond the North Wind was the third book I wrote for my kids. It is not connected to the other two, and in fact elaborates on a story in the Greek historian Herodotos. Before I wrote it, I told the kids, 'We have griffins and we have one-eyed Arimaspians: which ones are the good guys and which ones are the bad guys?' The griffins were unanimously elected as the good guys.
I wrote three other stories for my kids: two science fiction books and a medieval fantasy. The publishers didn't go for them, but the kids and I enjoyed them.