My gut feeling is that you should likely be looking at a 4-Runner, 8cyl.
The latest generation 4Runner is no longer available with a V8.
Surprisingly, the 4Runner and Highlander are very similar when it comes to ratings and capacities. The big difference is one(the Highlander) is based on a unibody car frame, the 4Runner is a body on frame based on the Tacoma. They both have the same 5k tow rating and similar payloads depending on trim levels. They both have 270hp V6's but the 4.0l in the 4Runner pulls harder with 278ft/lbs of torque vs 248 in the 3.5l Highlander.
Either vehicle will do the job but you will be at the limits with the full family on board. The Highlander is a more comfortable car-like driver, the 4Runner more truck-like but still comfortable.
One thing to keep in mind is the vehicle construction if you need to use a WD hitch, which you probably will. Unibody vehicles, like the Highlander, don't usually recommend using a WD hitch as they are not designed to handle the added stress. The 4Runner is one of the few mid-size SUVs to still use body on frame, although you will need to get a custom frame mounted hitch. The stock 4Runner hitch is cross member mounted and is also not recommended to be used with a WD hitch.
