OK we did it four years ago. My wife works for the school system so we are limited to summers. First make sure you have a working a/c. We didn't do any long hikes but seeing that you have a high clearance truck you can do what we did. Explore the back road network, get out do fairly short walks. Wear hats, we wear shorts but actually the brush and potential snakes make long pants more sensible. There is a spring at the east end of the park on the river which is a swimming hole. We've been there a couple of times but haven't made it there to the springs yet. I would not swim in the river, when the section you are looking at has actual water in it , it's muddy water and the Mexicans don't care what they pump or throw in it.
If you want to cross over to Mexico don't forget you'll need passports.
Drive the River road, that's a long day so carry drinks, snacks and probably a lunch would be a good idea. There are mine ruins, and a large ghost town that your child will really like. There are short side roads to the river. Very much worth the drive.
Visit Terlingua tho it will be mostly closed up, they have a historic graveyard and other places to look. The one open general store/tourist trap place (in the town not outside) had a lot of stuff mildly NSFW in large quantities. I guess that's the clientele they get in the busy season winter. So decide if you want your child in there.
There is a State Park: Big Bend River Ranch. It has an excellent cool temperature slot canyon right on the river. Maybe 1000-1500 feet do walk it. There was no river (dried up) when we went. It's called Closed Canyon on 170. Also there is a movie set town there were they filmed some TV and movies. Right off the road you'll see it. Stop there. There are some river access points where you can easily walk across from one country to another. No fences or Border agents in sight. Tho my wife says she saw a threatening sign.
Of course the best time to to hike is very early or later in the day but the park is big that it's hard to do. Drive up to the Chisos Basin, that's the only place you'll probably see a number of people, it's higher altitude and cool there.
Under no circumstances drive Black Gap road, it's a short body 4WD vehicle difficult 4WD road. Unless of course you are experts with the right vehicle, I understand it involves serious rock crawling. The rest of the roads are safe so far as I know.
We stayed at Study Butte right outside the west gate of the park, it was the only open RV CG (just a flat bare patch of sand) but with FHU. I understand there is a FHU CG in the park, stay there if you can.
Lastly shop at Alpine or Ft Stockton before you get there, prices are high at the park. There is a mini supermarket just south of Study Butte and there is fuel inside the park and at Study Butte. We planned for a week but called it quits after 5 days. It was hot hot hot. But we still had a good time. Don't plan on sitting in the CG the a/c might have trouble keeping up. And remember we are Floridians when we say that.
Have a good time, we did.
Arctic Fox 25Y Travel Trailer
2018 RAM 2500 6.7L 4WD shortbed
Straightline dual cam hitch
400W Solar with Victron controller
Superbumper