What do you want to know? (Until late last year I was a YA CM...)
It usually happens the first (full day) morning. It's just for 3,4,5 year olds to Come in explore the club. Often there's a programme e.g little einsteins, heffalump expedition. Its pretty quiet and calm. It's usually for 30-60 minutes. It is drop off but sometimes follows Open House, so you can go in together get used to the space then drop off for preschool fun. When it ends, they just open up to everyone so you want to pick up before if you don't want them with older kids.
If I kid gets upset or asks to leave they'll send a message to the wave phone, but they usually try and get them to stay around 20 minutes first.
That sounds like a great idea. My five-year-old son usually adjusts better when he can get some personal interaction the first time he enters a place. I know that that's hard to do when the place is open for everyone. But for the preschool gatherings, about how many kids do they usually have per CM? Or do the numbers fluctuate so much that it's too difficult to estimate?