English Translation
Because of the hot sun, they were tired.
日差し (sunlight) + 暑い (hot) are a very unnatural collocation.
疲れていました also sounds unnatural. It is more likely to be interpreted as “the sunlight was hot, and also they were tired”. In other words, the original Japanese sentence fails in connecting their fatigue to the root-cause.
My alternative translation is:
日差しが強くて、彼らは疲れてしまいました。(Lit. = the sunlight was so strong that it made us tired.")