What room type are you staying in? I will need to find accom for 5 next year and everywhere I have looked at has a maximum of 4.
We have stayed at MGM once, and twice at the Luxor.
It's a Deluxe Room. I booked it through Expedia and they would only let me book 2 adults and 2 kids BUT I called the hotel direct and they said it was fine to have 3 kids but the system wouldn't let us book it that way. I'm not sure if we'll have to pay extra for a rollaway when we get there. I'll let you know when we get back though.