Phrases like 'bit off more than he could chew' sprung to my mind.

Glad you both agree!

It's still, to me, a more interesting piece of work than Rowling;s Potterings, but the plot got quite plainly lost.

Although, I think the Potter stuff is a true children's series. Because it can be read by or to most children and they can get something from it. In contrast His Dark Materials is more a book for intelligent children and because they're intelligent, rather than because they are children. It has a sense of "worthiness" about it.