To book a family room for 4 when there are 5 of us?

[CollectingCoins]

Planning a one night city trip in UK. Finding it impossible to get a family room for 2 adults and 3 kids. Seems we will need an extra room no matter where we stay. Is it totally unreasonable to book one for 2 adults and 2 kids and sneak the extra kid in?

There isn't though. Do you think hotels insurance is invalidated every time someone pulls and brings someone back to their hotel? Think about it.