Which hotel is it? As you could try contacting the hotel direct and asking them if there's Queen bedrooms left. It may just be that TC's allotted amount of those rooms have gone.

It's the Aria and there are no queen bed rooms available- from their website.
Pablo- what exactly does your booking with Thomas Cook say? I just checked my bookings for another Vegas hotel and both had King down as a request. If you dont' have a booked that is specifically for queen beds than I don't think there is much you can do. However, there may be more leeway if you booked queens and its down in the paperwork, Again though, you would need to check the T&Cs. What exactly do you want form TC? If you want queen beds then it looks like they would have to move hotels. The Aria is a beautiful hotel( well I don't know about the rooms) and right in the middle of everything. You may end up regretting a hotel move.
In your OP you state you "believed" you had booked queen beds which would indicate that it is not stated in your booking. Why not go for something like they include breadfast in the deal?
It is a real pain if you have to use a roll out bed but if yu make the most of the Vegas experience you won't be in it much I also believe that the Aria's roll out bed probably isn't what you are imagining.
Vegas- a place I only went to as OH wanted to go and we have returned since because I wanted to!

I just tried a dummy booking with TC ( not for your dates) but they are actually very specific rooms you are booking. I do think you have a case.

What does the Thomas Cook Tour Operators invoice that you received shortly after booking have on it? It's the customers responsibility to check all information when you receive it, as per the terms and conditions, and notify them immediately if there are any errors.
What has your agent said about it?