She should protect them. However, if she’s not a good mom then she, herself may kill the chicks (but that’s not not that common). If she’s low in pecking order, she may not be very good at protection from others. However, it will be pretty clear right away as to how she’ll be with the chicks, and how protective she is against the flock.
we’ve given a broody day old chicks and she was great with them (she adopted 3 different batches of day-olds and was a great momma hen). In our situation, we put her in a large brooder, on a nest with fake eggs. Chicks placed under her during the night/ in the dark. Then the chicks were raised by her and stayed away from the flock for at least a week (smaller fenced off area of the run connected to the brooder in the barn). We would begin to let her venture among the flock around 2 weeks when chicks were bigger/stronger and listened to momma very well and she still had raging momma hormones and would essentially attack any other chicken that came too close to her babies. Because we had the ability to separate momma and chicks for a short time, this is what we did.