I have not written a childrens' book but I remember so many of them from my own childhood. Pictures are all-important. Wording should be simple and understandable depending upon age. By all means include a child's interaction with animal or bird life. I am not too fond of cartoon characters, but drawings are quite effective too.
As to short stories, it depends upon the circumstance. One of my short stories was recently read over the local station of National Public Radio. They requested it be at least 40 minutes long, which means something close to 5000 words. When I give a talk they expect at least 30 minutes allotted time, or about 3000 words.
I like brevity, and avoid purple prose, so my preference is to keep a short story short, no more than 1500 words.
Hope that answers your question.