Lots of kids your sons' age play with their private areas and even with their excrements, without it ever progressing into anything as bad as molestation. Kids enjoy "playing doctor" and exploring things "down there." They're simply curious and can't understand why it should be wrong.
Personally, I would not discipline your son(s) for their curiosity, but I would set limits as to how far they can go in their explorations, for example:
1. No playing around with excrements, for reasons of health and hygiene.
2. No playing games with another person's private area against that person's will - explain that a private area is private and should always be respected.
3. No inserting of anything into a body opening, b/c of risk of injury.
Hopefully, this would put an end to the grosser games, while still allowing your children to get acquainted with their bodies.
BTW, I think that the more they are led to perceive such games as "forbidden," the greater their fascination will be, and the greater the risk of an ambivalent, unhealthy attitude to their own sexuality will be.