I agree. Perhaps child protective services needs to get involved.
By your count you have a 3, 4, and 9 year old who are all doing sexually improper things. I would be very concerned at this point. In fact, since all of your children (any others around?) seem to have this problem. I would think that professional help might be needed. Some adult in the family situation seems to be failing in their responsibilities, and its possible that family services may need to be brought in if this continues.