if she is acclimated to the heat and he knows what to look for as far as heat stroke, then I wouldn't worry as much. As you said, she wants to go. Does he amble along as he walks? Or does he hike like it's a military drill? That also makes a huge difference.
She's turning eight. Sounds like she's been doing it for quite a while now and is accustomed to the heat. She hasn't had a heat stroke yet.
Like I said, make sure she stays hydrated, print out the heat stroke information and you are good to go