Holmes' disdain for the remarkably long-lived works of author Stephen Cosgrove and illustrator Robin James is perfectly understandable. I mean, the thing is, kids have no taste. Certainly no discretion in the way of literature. It's only as we grow old that we learn to avoid things which have the moral printed right there on the cover.
That's literal, in the case of the Serendipity books. On every cover is a helpful call-out box titled "The moral of this story is:" For example, the moral of The Wheedle on the Needle is, "Cooperation can solve almost any problem."