thats is normal there just experimenting each other bodies. just make sore they are not doing in front of any other children .
At this age it's normal. Don't be too harsh on them, just nicely explain that "we don't do that." They are learning about their bodies and the fascination of it all. You don't want them to feel ashamed of their bodies. Maybe when you catch them, remind them that we can't be doing this, let's go play, just redirect what they are doing.