Hey Fanger,
I have a lot of experience with this. I'm a father of 3 young girls and we travel a lot. This topic also comes up on the Hotwire / Priceline forum that I administer (
www.hoteldealsrevealed.com/forum).
The rules of Priceline are that the room is guaranteed to accommodate a maximum of 2 guests. In most cases (but not all), the hotel will accommodate your request. It depends on when you arrive at the hotel, their vacancy rate, etc. and it's a risk you run. I personally have had a hotel not able to accommodate me (my kids slept on the floor but they were only 4 years old so no biggy) and have heard of others with similar experiences. But more often than not, you'll be fine.
As previously stated by others, there are fire regulations that control the maximum occupancy of a particular room. However, this risk if this is even smaller and very rare.
I'd strongly recommend you consider Hotwire if you're concerned about the risk. For me, I require the Priceline price to be A LOT cheaper before I'll take a risk. It's not worth $10 or so in savings. $25 or more per night... okay, I might take the risk.