"Art flourishes where there is sound critical judgment to examine and appraise. The critic must, first of all, have a real point of view about his subject. The essential point of view grows out of acquaintance with the best children's books past and present, and also with the world's best literature for everyone. This point of view - this measuring stick - must also bear some relation to children themselves and their reactions to books today. The critic should have experience of sharing books with children or of seeing them choosing and reading books for themselves. It is a truism - and yet it does not seem to be generally understood - that criticism is just as importantly concerned with pointing out excellence as weakness.
. . .
Comment on children's books is valuable in exact proportion to the judgment, honesty, fairness, and skill expressed by their critics."
Well, that's a pretty good starting point. And a pretty high bar. In summary, a critic should
Measure a book against some stated standards
Care about the book
Be knowledgeable with the body of children's books present and past
Be widely read in general literature and history
Have experience reading to children and how they respond to stories
Have seen how children pick and choose books for themselves
Offer balance with as much empahsis on the positive as on the negative
Judge the book and express that judgment felicitously