In my experience, as long as they have some place warm, dry and safe to get to they will be fine for some outside play on sunny days. Mine are usually raised with a hen, but I have had several that have been abandoned around 3 weeks to make their own way amongst the flock. Two of these 'orphans' are still in my flock; one is now a beautiful 23 week old rooster, the other is a 5 week old hen that has done just fine in temps as low as the 20s free ranging during the day.
That said, chicks and chickens need to acclimate to varied temperatures and always have a haven to warm up if need be. They aren't nearly as fragile as one might think, but if put out suddenly or without proper shelters they risk sickness and death.
If the sun is shining and there's no wind I'm sure they'll enjoy some outside playtime.
Just stay with them constantly and watch for signs of being too cold - huddling together, peeping loudly. That means it's time to go back in the brooder under their heat lamp(s).
Mine were outside on nice days at 2 weeks old.
I'e read they can go out as early as one week (or day one with a mama) and they will be fine as long as they can get back to the heat perdiodically. Mine were out in the coop at two weeks and it would get under 40 degrees in the coop and they'd run around and play in the non-heated area and then warm up under the lamps. Mine are now 5 weeks old and they started going outside to free-range at 4.5 weeks old.
It's my belief that they should get out as early as possible, so go for it! good luck to you!