The climate over here may be different then, there is little danger of heatstroke as long as they have plenty of shade, and permanant access to water. they will certainly be better of out in the run than in their hutches. The guinea-pigs live in the shed all of summer, spring and autumn. Given the choice they prefer to wander across the lawn to the living room. It may be because it is how they were raised.
I forget that other places are not the same as here, but long periods of time on the grass aren't too much of a health risk here, and as long as they are properly monitored I don't feel it is a problem here.