Chickens are precocial birds: when a chick hatches, it has feathers and is capable of feeding itself if it has access to food. The only thing the broody hen does for the chick is keep it warm and protect it from danger as best she can. She will also make a special clucking sound to show it where food is. You will be amazed at how fast a chick will make a beeline straight for the hen when she makes this sound!
A broody hen usually stays in the nest for couple of days while the whole clutch is hatching, so she may not leave the nest with her hatched chicks for some time. This gives the chicks that have hatched a time to dry off, rest and gather their strength. They can live without food and water during this period off the last nutrition they got from the yolk before they hatched.
How many eggs were under your broody?
Just make sure chick starter feed and water is placed close to the nest, and make sure the chicks can get out of the nestbox and then back in when the hen tries to lead them back at night.