5 year old boy and private parts

My 5 year old son was playing at my house with my niece (4) and nephew (5)upstairs in his room.  My mother was watching the kids while we were out. When my husband, sister and my older son (9) came home, my son went upstairs and found my 5 year old nephew wearing a pullup.  45 minutes later my sister called me telling me that her 2 kids (my niece & nephew) said that my 5 year old was touching their privates.  My son denied it at first, then he said yes, he had.

5 months ago, my niece (same one) and my son were in his room playing and my sister went upstairs and found my son checking out her privates.  

Is this normal behavior for my son?  I am very concerned.

HELP!

[Kevin Kennedy, Ph.D.]

Such curiosity is normal for the age, and limits must be set on the behavior that surrounds it. That is, it is not permissible for the children to be undressing with one another and touching one another's genital areas. Children so young really should not be left alone in a room without adult supervision. By nature they are impulsive and they need the help of adults to control those impulses.

[Person of 12]

I've recieved the same problems!

Just recently, my 5-year old son, and my son's 5-yearold friend, experienced a problem. my son's friend, was coming over to spend the night, and i decieded, to put them in the same room, in case they got scared. so i put them to sleep, and told them in be good, and left. everything was going good, when the next morning, i went in there, and they were both naked, laying verticul to each other, with his friend on top, licking his 'parts', and him licking hers.