<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by The _Banking_Scot:
The Mandalay Bay also asks for a key for each guest (even children)when entering the pool area and they swipe it to ensure it is a current key ( so you cannot use a key from a previous stay)</font>
Just to confirm what was said, Mandalay Bay does require a key for each guest,
regardless of age. They make this very clear, so families checking in are advised to get a room key for each person staying in the room. This can be a problem if more than 2 adults are sharing a room, as they charge $35/night for each person over two in a room. On nice days, their pool area is absolutely packed, so they need tight enforcement in order to not turn away registered guests.
Reporedly, Monte Carlo is less strict, and allows a group in with only a single valid key.