Seriously, you will be making a mistake to 'sneak' chicks under broody hen at night. You should wait until closer to the 21 days, but then take the egg and bring at least 2 chicks out to her. Let them interact. The chicks do not have to be only a few days old. The hen will really not know the difference if the chicks are a week or so. DO NOT put them under a hen in the night.