70 is still cool for new chicks. 2 week olds have fluff not real feathers to insulate and help keep their body temps regulated. I would go out with them and keep them out for only a short time then right back in the brooder. A momma hen would take them out and then call them back in and hide them under her wings for warmth.
I think we will just take them out for a few minutes to let them play and scratch around a bit. I wouldn't want to leave them out for very long so I think it sounds like they should be okay for about 10-15 minutes.