I do a lot of trips from from VA to IL and take my 95lb lab mix with me everytime. What it really comes down to is your dog's personality. I usually fold the 60 portion of 60/40 split seat down and lay down a tarp then her bed on top (the folded seat gets a blanket). Till we get on the highway my lab usually has her head out the window (tongue sticking out and all
), but once on the highway she's usually curled up behind the seat that's still up. She's not one to come up to the front, so saves me the trouble of having up put up a gate and all. The seat that is still up should be plenty of room for your kid (even with a child seat if needed).
Just another bit of advise...make lots of stops, take pictures and enjoy the road trip