When you check in, tell them the two other people (if they ask for the extra cash) are children under the age of 10 -- and can you please get a rollaway for them?

That might work, and they may not charge you anything extra for that. However, if they don't bring up the charge at checkin, they may not have noted it. Just ask for two beds and see what happens. You can always check out the room first (you should anyhow!).

Not to hijack threads, but I have a question. If we arrive at a Priceline-paid hotel, and the hotel is awful (i.e., a 1* when it was supposed to be 4*), does calling Priceline mean they will find us a better one right away, and transfer the cost?