The chicks will need to stay inside in a brooder for the first 6-odd weeks and after that you can move them outside, but don't put them with the older hens yet. First put them in a separate area where they can see each other and let the hens get used to the chicks for a few weeks before letting them mingle. It is usually recommended that chicks be introduced when they are around the the same size as the existing flock. Even if you only have 2 hens there is still a risk that they may injure or kill the chicks. By letting them get to know each other first you will will ensure that the later proper introduction will go much smoother and this applies any number of birds.