Crew cab truck at a minimum. I've got an 8 year old, 5 year old and 21 month old and 3 small dogs. We have a crew cab 2500, but our next truck will hopefully be more of a megacab style. It just gives more interior room and allows the rear seats to recline which will be more comfortable for longer trips. I'm tall, 6-04, so even with the crew cab we have our youngest behind my wife's seat because there isn't quite enough room for the rear facing seat and my seat all the way back. Keep in mind that a pickup will allow them to touch/pinch/scratch/steal from/etc each other the entire drive.
Hopefully Ford and GM offer a mega cab style pickup in the future. It sure is nice. My 8 and 5 year old are already very tall for their age and will continue to need more and more leg room.
Since you are buying a truck for camping, please start off right and get a heavier truck. The kids will continue to grow, the stuff you bring camping (bikes/toys/wood/grills/etc) will continue to grow. All that stuff adds weight. A half ton might get the job done, but a 3/4 or 1 ton will definitely get the job done and won't be nearly as close to maxed out while doing it.