The best way is to wait until they are the same size as the adults. Then, put them in the coop at night when everyone's sleeping. Keep an eye on them to make sure nobody is getting seriously pecked on.
Another option is to put the chicks out in the coop/run area but have a fence of some sort between them and the adults. Let them stay by each other for a week or two (longer is usually better) so they can see and hear each other. Then, let the chicks with the adults during the day in the run or free ranging for another weekish. But keep them separate at night still. Then take the fence down and usually everybody is fine together.