You can make reservations 120 days out and you have to secure your ressie with a credit card. I believe you can cancel up to 24 hours before and not be charged. I also think it is only held once or twice a week. I know it is on Thursday. Look on the Dis Home Page for more information. My dd's are 4 and almost 7, we will be going in June.
I copied this info from the Dis info it is under Dining/Restaurants
This awesome adventure is being offered Mondays and Thursdays from 10:00 a.m. to noon, and starting April 3/02, it will be offered on Wednesdays as well (available to book now). You can book this 120 days in advance by calling 407-WDW-DINE (407-939-3463) to do so. The cost is $22.95 per child, a credit card is required at the time of booking, and parents must sign a waver upon arrival. Cancellation policy is 24 hours in advance and there is a $15 "no show" charge per child.