With a 3900 lb. axle in front, you have over 700 lbs. to add to the axle. A properly setup WDH will get the weight of the tongue back on the front axle. if you are worried about your family overloading the axle, I don't think there is a concern there. If you and your hubby are sitting in the front seat, that is an additional 395 lbs........but y'all are not over the axle. The front passenger seat on super or crew cab pickups sit about 1/3 of the way back on a truck, so approx. 2/3 of that weight will go on the front axle (263 lbs.), and 1/3 will go on the back axle (132 lbs.) Likewise for your daughter and dog, since the back seat is about at the 1/2 way point, 50% of their weight will go towards both axles. Anything loaded to the front of the bed will be about 1/3 to the front axle, and 2/3 to the rear axle, and anything directly over the rear axle is going on add weight to the rear axle. That is why a trailer tongue causes so much trouble without a WDH.......it is adding weight a couple of feet behind the rear axle, taking some weight off of the front axle, and adding that to the rear axle. As you may see from my spreadsheet, adding 680 lbs. to my hitch takes off 280 lbs of weight on my front axle, and my rear axle goes up 960 lbs. If you subtract the 280 from the 960, you get the trailer tongue weight.....680 lbs.
So to make a short story long which I have already done
, if you get your WDH setup to put the original weight back on the front axle (and maybe a little extra), you will still have somewhere in the vicinity of 700 lbs. more to add there before you are maxing out the axle. Without a WDH and the bed of the truck loaded heavy, you could max out the rear axle, because of the leverage affect on the trailer tongue weight, although a 400 lb. tongue weight is not as bad on that as my 680 lb. tongue.
Before heading to the scales, I set up my WDH in the driveway using the fender measurements including in the setup instructions of most WDHs. My front fender dropped 1/16th of an inch when the trailer was hooked up over not being hooked up, and the rear fender dropped 1 1/16th of an inch. With those figures, I figured I had a little extra weight on the front axle which turned out to be an extra 40 lbs., and a lot of the trailer tongue weight was being carried by the rear axle, which is meant to carry weight.
Hope this helps.