So pretty much how you can trigger MASS CHILD MAKING EVENTS: get a bunch of 3-star S.Desire yukkuris, gather them together, grab them and keep rubbing them with the broad fondle hand. Just let it go its course - you should notice some of them wiggling up and them really hard which means they're refreshing. When you hear stuff like "dekitayo! Akachan dekitayo!" or "ochibichan" then that's it, they're preggers. This method also turns C Ranks into S Ranks with some ease so it's a win/win (the refreshing animation ain't all that interesting really).
Set the pregnancy possibility to highest, as well.