My twin boys turned 3 ten days ago, so they're pretty close in age to your pair. I have tended to use their individual reactions & boundaries as important deciding factors. So when it comes to what do you do if one is giggling & saying it's okay, to me that would be important feedback and it would help me organize my own reactions, and separate what is an issue for me versus what is part of the actual present moment. (This is necessary for me in lots of areas while parenting, including this one.) So being present with myself (mindful presence, an "Observer Self" that can see my reactions & notice my thoughts about them & all the resulting feelings) is an important piece, not just the question of "what I do when...." or "what I do to stop them from...."
So in that situation (giggling receptivity) I most likely would wait & watch. If I had concern specific to the situation (I am considering your example as one out of MANY scenarios, not the main/only situation, so I am not really focusing on the mouth/bottom contact as the main issue, you know?), then I would voice it. So I wouldn't rely only on their signals for whether or not to speak up about a separate concern (such as hygiene/germ transmission, as mentioned by a pp) but I try not to be pre-emptive about that kind of thing, either. Or simply just to be really aware of what is fueling my impulse.
In my experience, emotion-fueled (usually fear-motivated, for me) attempts to stop or redirect behavior tends to trigger counter-resistance in the child, because of my initial resistance. Connecting to myself & then to them/their experience is something that comes first when things work well. This is why connecting to the actual present moment & to what is real for the children involved (connecting to their validity) is so helpful, whether the behavior is problematic or whether you ultimately discover that it's not actually a problem. Otherwise, defensive responses get triggered as the child reacts to the negation they experience with our resistance or our attempt to stop behaviors.
In the end, I think my main practice is to support them in noticing & respecting each other's "messages" about personal space and boundaries. I did go through some personal discomfort or anxiety, mostly due to "What if?" kinds of questions internally that triggered an impulse to stop or squelch the behavior at a very early stage (and I certainly wasn't aware of any "What if" questions until I really was willing to tolerate my discomfort & explore it; the only thing I experienced consciously was an impulse to stop, distract from, redirect.)
We haven't really had an issue with the behavior developing into a problem or really even persisting. Most of their interactions around genital curiosity tend to be verbal, now. (Statements about the other twin's penis and various parts, rather than touching or pulling.) They were quite a bit younger when the potentially more "intrusive" stuff was going on (and triggered my uncertainty, discomfort, concern, & fear), and they aren't doing a lot of exploring or touching at their current age, is what I am saying.
I don't know if this is because I didn't initially set up a control dynamic or not. I do know that when I've felt that is the case about something (that injecting control or struggle or resistance ultimately reinforced certain patterns of behavior) that it is helpful to view the behavior as a signal (perhaps a signal of what is out of whack, a problem with control) rather than as the problem itself, and to address the underlying issue. What I mean is that problems don't have to STAY problems, and can self-correct in ways similar to there never having been a problem, even if it's an "Oops, I wish I'd responded differently, NOW look what's happening" situation.
Obviously, this is all personal stuff related to me. I share it because it's a definite part of my process, and it has been true for me, but it may not be relevant to you & I don't intend to suggest that I assume that it is!
Edited by AmyC - 7/31/11 at 5:35am