At 21'C I'd be very surprised if they came to any harm. If they can have their heater so they have the option of warming up, they'll be fine. If they were under a broody no doubt they'd be outside briefly from a day or two old, but have mum to snuggle back under.
Keep an eye on them, if they spend a good bit of time fluttering up and down and scratching great. If they spend the day in a clump under e heater, take them back inside. I like chicks to have space and be outside if possible, if weathers nice. Maybe they're just as happy in the home that they know, but I like getting them outside.