NIMH or LiPo for running all day in summer heat?

Hi r/rccars, this summer I'm going to offer RC cars as a summer camp activity. I'm aiming to keep a few cars in operation for eight hours a day. They'd be outdoors with peak temperatures of 105F / 40C. I'm looking at 1:24 cars with 1S batteries right now. This would mean having batteries constantly charging and discharging, with drive times of 15-20 minutes – full to empty. I figure that's eight cycles per day per battery pack.

My question is, which is a better choice for this: NIMH or LiPo packs? Specifically I'm wondering if the constant running and heat and increase risk of fire with LiPos.