@Paola2127 A few thoughts
1) You can link the calendars of the two listings so as not to get double bookings. You then make the smaller listing unavailable in, perhaps, high season to force the bigger listing to be booked. The problem with making one listing bookable on demand is that rejecting booking requests will drive your search engine position down.
2) You can say the place is not suitable for certain age ranges but guests do seem to ignore this.
3) The cancellation terms are set already and are either flexible, moderate, firm or strict. You set this in the listing (under rules I think but might be availability). The flexible allows cancellations up to and within the holiday - Strict means you get at least 50% or more if the guest cancels. Obviously you will get more bookings with Flexible but they are not certain bookings...