First option might be a bedroom suite (essentially two bedrooms with the divider removed to make one room). You will have only four beds, but there are two armchairs for the extra person (have to be pretty small children to share a bunk). You also get two private restrooms with two showers. Problem: short one bed, and its very, very expensive.
Second possibility - book the family bedroom and one Roomette. Six beds, but two of them are child sized berths, and no in-room restroom facilities (but they're right down the hall). The Family room won't work by itself, but you need only one Roomette in addition to have plenty of space for everyone; If you booked Roomettes alone you would need three of them. The Family bedroom has four Roomettes right outside the door, so you will all be close together at least.
Final option, and which may be your best bet, just go coach. It's perfectly workable for just the one night, economical enough, and you can easily have five seats together.