My guess is they are aware of any type of "scam" and have ways to prevent it. Either by charging for a "lost band" and/or being aware of the larger groups of kids and knowing that it would not be that unusual to try to sneak one more in. Be aware that most hotels will kick you out if there is any sort of problem and they will use the fact that you had more kids in the room then the reservation said as more leverage. If you are as quiet as a church mouse you will probably have no problems.