Dear Ms. Delauder,
No, this is not normal behavior and indicates that inappropriate sexual behavior is occurring between the two children. It's not possible to say with certainty what might have happened to each one of them in the past, but they either have been directly sexually mistreated or have been exposed to sexually stimulating experiences, either in person or via movies, videos, etc. It's likely, unfortunately, that some direct sexual abuse has occurred.
Without question these children require immediate professional intervention. Helping to obtain it is the best thing you can do right now for the kids.