Just got through doing what you are doing. Most 1/2 ton trucks are not keen for a larger trailer but at 21" you may be OK if you check payload as has been suggested. Look at the max tow as well but the engine in your truck is also a consideration regardless of specs. I had a 2013 F150 with the 3.7 Flex and while it had pretty good horsepower, lacked torque and didn't make a good tow vehicle; in addition the suspension and automatic transmission would not hold up towing very well.
My first thought was to have you folks buy a nice used trailer with only a single queen and then go to the army/navy store for a 3 person tent but I guess you have seen enough cookie-cutter answers. Since my wife and I are the only users, can't give advice on how to best stash kiddos but one thing to look at is that if you get a bottom line trailer; be it used or new, the A/C may not have enough BTUs to keep you all cool in warm weather. Some low end trailers have really low end A/C. Another issue is that with 5 folk, where do you intend on using the trailer? In an RV park setting? In the boonies or someplace without water or sewer hook up? If so, many smaller and less expensive trailers have dinky water, gray and black tanks. Something to factor in.
In ended up buying a 2011 Sunnybrook Harmony 21FBS whcih has a regular size queen walk around and a full bath with a shower door. Best features are nice robust A/C and relatively large tanks. My trailer is 22 ft and an easy pull (very light) and pretty decent quality. If you go used or new, look at things like quality of plumbing fixtures and insulation. These things are not often mentioned but can be make or break when you are out away from home.
wiredgeorge Mico TX
2006 F350 2002 Keystone Cougar 278EFS
2006 GL1800 Roadsmith trike