Firstly, stay calm. Reacting with anger or shock might scare the child. Just firmly but gently tell the daughter that this is not acceptable behavior. For example, you can say 'Sweetie, we don't do that. It's not nice.'
One possible reason could be a form of unusual or inappropriate play. Maybe the daughter saw an animal do something similar and was mimicking it without understanding the inappropriateness. However, this is a very abnormal situation and proper communication and guidance should be given immediately to correct such behavior.
Well, when you catch your daughter in such a situation, don't be judgmental right away. Remember, she might be exploring her sexuality, which is a normal part of growing up, although it's a very uncomfortable situation for you as a parent. You could start by providing her with proper sex education if she hasn't had enough. And also, make sure she understands the importance of respect, both for herself and others.
I've seen a mother cat that had just given birth. In the first few hours, she was constantly licking her kittens all over, including their genital areas. This licking serves multiple purposes. It not only cleans the kittens but also helps in the bonding process between the mother and her babies. It's an essential part of the early care that mother cats provide to their kittens.
Definitely not. Children's literature is meant to be a source of positive influence for kids. A'mom licking daughter story' sounds very abnormal and goes against the norms of healthy family relationships and good moral values that should be instilled through literature for children.