You should assume that the rules will be strictly enforced. Moreover, the setup of many European hotels makes it difficult/impossible to sneak additional guests into the hotel as the front desk sees everyone who enters the lobby and walks toward the rooms. This also isn't something you want to teach your kids.

Your best bet is to contact the hotels well in advance, being honest about the numbers and ages of your party. In some hotels, they can set aside an appropriate room when you make the reservation. In other places, your only option might be to pay to guarantee a larger room or suite.