Get a rear living with a queen fold out bed (our couch folds completely out using all cushions to make queen size bed and it's not too bad according to my son and daughter-in-law) and then opt for booth dinette vs table/chairs. Put the 6 year old on the booth dinette and the 2 teenagers on the couch bed. Or if you prefer the table chair setup, get the theater seating in place of regular recliners (this is what we have). It leans back nearly flat and is very comfortable. My 21 year old has slept on it a few times with no complaint so I'm sure your 6 year old would be ok there for short trips as well. Also, if you prefer using a blowup mattress vs the sofa bed, a triple slide unit offers ample floor space.
We bought ours after the kids had grown and left home so we basically got what we wanted but they are welcome anytime to go with us. Had they still been at home, our purchase may have been different as we may have opted for a bunkhouse. It sounds like you would prefer the added living space since the entire family is not with you all the time so I say get what you want. They will always manage. Ours has storage in the living area that is completely empty most of the time so there is lots of places to put the kids stuff to keep out of the way.
I look at it this way, my sister has a 27' travel trailer with one slide and a set of bunk beds in back by bath. They take many family trips together and they have a 16 year old daughter, 22 year old son, 25 year old son and 25 year old daughter-in-law. They have a blast camping together! If you can fit that many grown people in 27' using 2 bunks, dinette and couch then I can for sure enjoy my 37' three slide unit and make it work when needed. That's why I say get what you want. It will all work out ??
