I try and feed a mostly gorage and "weed" diet in the summer so my rabbits get daily greens as soon as its long enough to pick and I start putting my house bunnies out on the lawn, including kits.
The earliest age that I put the babies out is 2 weeks old and when the temperature is above 20'C (75'F, I think) I bring out the nest box and a handfull of grass hay as well.
I never put babies out before noon as early morning grass has a higher simple sugar content then later in the day and I closely monitor their poops to be sure all is running smoothly.