Not sure what you mean. Do you mean a permanent move out or a short visit? And what temperatures does "warm and nice" mean? Will they have protection from wind?
A broody hen typically has her chicks outside scratching and pecking at the ground just a few days after they hatch. Many people that haven't seen it would be amazed at how long these baby chicks can stay out even in fairly cool weather before going back under Mama to warm up. There have been photo on here where a broody hen took her very young chicks for a walk with snow on the ground. Obviously the warmer it is the longer they can stay out but, as I said, some people would be amazed.
I'd wait until they are at least three days old, they tend to stay under a broody hen a lot until they are about three days old. But if you take them out where they are protected from a wind they will enjoy the outing. Keep them in some type of enclosure, usually netting or a small fence. They can be really quick and hard to catch. When they get cold they will tell you, giving a really plaintive peep that clearly tells you that something is wrong. As long as they are running around they are happy. Take your cue from how they act.
My chicks are typically fully feathered at 4 to 5 weeks of age. I've had chicks 5-1/2 weeks old go through nights in the mid 20's Fahrenheit (-4 C) with no heat source. The coop they were in had great ventilation up high and great wind protection down low where they were. They had been exposed to cold weather so they were acclimated. If you are talking about moving them outside permanently I'd want to know the coolest temperature they will see and make sure they had good wind protection.
If it’s summertime, I usually move mine out at 3-4 weeks, or even earlier if it’s hot enough at night. I make a safe place for them at the bottom of the coop so my older hens can see them, but not get to them.
I start letting them visit the run as soon as possible, in a pen when the hens can’t get them. It makes their move outside a lot easier.