We have 4 - 13,10,7,7 - and we always book apartments or houses - minimum 3 bedrooms, but 4 if possible or if we're going for longer than a few days.
DTs will still share a bed, but they mess about if they do, so we prefer to find a place with 2 singles. (Often it's a king bed which can be separated into 2 singles.)
The older two either share a room in separate beds or have a room each.
It takes a fair amount of hunting around online to find a good property, but we've never not found one and had to discount a location because of it.
What kind of places are you looking? I will admit that we don't do beach holidays - usually city breaks - maybe that's the difference.
In the last 5 years we've done Barcelona, Tuscany, Austin, LA, Washington, Orlando, London, Edinburgh and Cornwall and never had a problem getting somewhere.