8 year-old sexual behavior towards sister, what should I do?

[uraqtpie]

Over the weekend I wasn't feeling well so I spent most of the day in bed.  My children's father was at work so I tried to balance household stuff and getting better.  So anyway, after a quick nap I decided to get up and get the children dress so they could run to the store with father when he got home.  As I walked into the living I was shocked and heartbroken at the same time to see my 8 year-old son with his pajama bottoms and underwear down and my 4 year-old daughter's skirt and underwear pulled down.  My son was crouching behind her, like he was trying to get down to where she was standing.  I did not see physical contact, but what I did see has left me confused and saddened.  My children never exhibited this type of behavior before.  My son doesn't even like to change his shirts in front of us, let alone this type of behavior.  I told their father about it when he got home, and my son soon came up with an alternate version of the events.  Their dad essentially blames me because I watch the news and that somehow triggered this whole thing.  Please can someone tell what I can do for my son and daughter ?

[margypops]

Certainly it would be a good idea to make sure he hasnt seen or learned this behavior, from somewhere or some one else or seen magazines or PC id you ask him pertinent questions about why he did it, perhaps supervision now is the answer, is he unhappy in any other aspect, does he do well at school .