Why did they spend 2 weeks in a nesting box, why do you want her to keep taking them back to the nesting box, and why do you think going outside is dangerous for them? At 2 weeks they should be outside experiencing the world with their mom. Is your run predator proof? If it's not safe for the chicks because of predators, then it's not safe for the hens either, and you should predator proof it. But if it's safe from predators, then it's safe for the chicks to be outside. It sounds like you only have one other hen besides the broody and the chicks, so that makes things easier. The broody should be more than capable of keeping the other hen at a distance. I let a broody raise chicks with the flock every year and by 2 weeks they are roosting up on the big roost with the rest of the flock at night (under their mom), spending all day outside in the sunshine, enjoying life. As long as they have their mom to warm them up periodically, and you're not in the dead of winter, they'll be fine, they won't be too cold. One year my broody hatched chicks in a snowstorm in April. It was in the 40s when she took them outside. They were fine. Give your little family some freedom to spread their wings!