I work in a hotel, not Legoland unfortunately 🤷🏼♀️ Just ring and ask, or book 3 kids and an adult, then ring and say you noticed it's wrong on the confirmation, booking systems can only do what they've been programmed to do online but usually a human can override it. Though you do run the risk of a no if they're strict about it. But just arriving with an un/incorrectly booked person runs the risk of being told no and you have to have an extra room on the day too.
I could put 10 people in a max occupancy of 2 room if I wanted to, it would give me a warning but let me do it, but you couldn't online.
It's likely that it's because the beds aren't really suitable for adults and they'd be at fault if they let you book say 4 adults in that room but 2 beds are smaller and not really suitable for an adult, they'd be miselling the room.
And it is actually a requirement (maybe legal or an insurance thing, I'm not sure on that bit) that hotels have accurate numbers and contact details for each room (the lead booker, not everyone) and no, the fire service won't just take staff's word for it when a building is on fire and there's potentially people in there that there's no one in that area etc, people wander around in hotels, people bring people back, people don't come back because they're in another hotel with someone, people room hop. Unfortunately I've been in this situation and they do a systematic search of all areas, they did ask if everyone that was booked was accounted for (they were) but they didn't just take my word for it, they looked anyway, but if someone wasn't accounted for they'd have known they were potentially actually looking for someone and where to start.
People do it all the time - do you really think staff kick you out if you bring a one night stand to your room, for example?
Well I have, and I've also turned a blind eye, depends on the situation.
One person booked and they try and sneak another 3 in late at night and pissed and I clock it? No, because chances are they're going to be noisy, piss off other guests and then I'm going to have a fight on my hands getting them out.
This situation? I'd just explain the beds aren't really suitable for adults, but change the booking to an adult but if it attracted an extra charge then I might waive it, depending on the attitude of the guest tbh. I'd only then be bothered if they complained about the bed not being suitable or wanted extra stuff or left a shitty review after being told about the beds. But that does happen and as they say, no good deed goes unpunished!
It's not about being the staff being a 'party to deception' it's about giving good customer service and understanding different situations and reacting accordingly.