We do monitor every book that is submitted, and it must go through a review process mainly to be sure that what is being submitted is consistent with what is being offered on Amazon. If anything is offensive or highly controversial, it may be rejected as it might be difficult to get the kind of reviews you would want for those topics.
Also, there must be at least as much unique written content as you would find in a children’s story book. (Most children’s books are accepted BTW.) Therefore, most coloring books, puzzle books, journals, logs, activity books, things of that sort that we consider “low content” or with repetitive text on most pages may not be appropriate. If a story accompanies the images submitted, or if descriptions, explanations, a unique tip, quote, or affirmation are included per page, etc. it probably will pass the test because of the amount of unique content present.