It's not that bad!
If you put the chicks under her and she seems to accept them, adjust her position for them, leave them under her, all is probably well. When I've done it, I've watched for a minute or two, certainly not all night. In the morning you will be able to tell easily enough if things are OK, they'll be under her, climbing on her, pecking the ground near her, etc. Once a mama accepts the chicks as hers, she will ordinarily keep them close by during the day and cover them at night for a few weeks. Then they will begin to wander further and further, and she may peck at them to tell them to get lost, but not viciously. Then she'll just go back on the perch and they'll likely hang together, maybe on the roost, maybe in a corner or a nest box til they're ready to roost.
I had one broody who did not accept chicks I'd bought. I foolishly tried to give them to her during the day. They seemed fine at first, but an hour later the chicks were in a far corner of the coop and she was on her broody nest. She hadn't harmed them, but obviously she'd kicked them out. If I'd been wise enough to wait for nightfall, I'd probably have had better luck.
eta I've always had my mamas raise the chicks in with the flock -- I don't want to have to integrate them later or finish letting them grow up separately, after she is through being a mama. The roos if anything have been protective of chicks, and the mama has quickly run off any hen who comes too close.