When can baby guinea pigs go outside?

I have two male guinea pigs living in outside hutches, and usually let them run around my back garden as it's huge. I have a female guineapig living inside, with her three babies. The babies are running around, eating etc and seem perfectly fine. So i was wondering, since it's quite a warmish day, do you think that it would be ok for them to be let outside in a playpen for an hour or so? Obviously seperated from the older males!