First things first you shouldn't be letting children that old shower together that are of the opposite sex. Your son is not too old but your daughter is. It is normal for children to be curious about each other's bodies but as a parent it is important for you to teach them what is and is not appropriate behavior.
However, the fact that your daughter was simulating a sex act raises the flag that she may have been sexually abused, have you talked to her about whether anyone has touched her or where she learned what she was doing from?
It may not be abuse, it may be what she is watching on TV. With children you have to be very careful that they are watching age appropriate material as they are susceptible to imitating what they see on TV.
Spanking them may have been your first reaction but it may have scared them into talking to you about it. You may need to apologize to them and explain why you reacted the way you did and explain to them that what they were doing was not a proper way for brothers and sisters to act toward one another.
Be very calm when you talk to them and be open to what they tell you. Also, if you do not feel like you are getting the truth out of them you may need to get a professional to talk to them such as a psychiatrist. Sometimes kids have a hard time telling those they love the truth if they feel it will get someone else in trouble.
Here is some information that may help you in asking about the sexual abuse.:
You are very welcome and don't be too hard on yourself you reacted out of shock and suprise because these are your babies. I truly hope it was just something she saw on TV and hope you get to the bottom of the situation. I wish you and your family the best!