Wait another day before getting her chicks to raise. It is unusual for chicks to hatch a day or 2 late under a broody, but it can happen. I've had a batch start hatching on day 22 and finish on day 25 under a broody once. If you'd live for the hen and chicks to sleep in that nest box you will have to construct a ramp of some sort for the chicks. Or you can put a "broody box" on the floor for them to use at first. If you get her some chicks to raise wait until it's dark before slipping them under her and remove her unhatched eggs at the same time. This way she will wake up to the chicks and be more accepting.
You can let the hen raise her chicks with the rest of the flock, or keep them separate at first. Ideally she should stay in with the flock though, so they can get to know the chicks and accept them as part of the group from the start. There is a small possibility that the older chickens may harm or kill the chicks though, so keep an eye on them at first and be prepared to move them if needed. If you do move them, keep them separate only for the first 2-3 weeks, so the chicks can get stronger and the hen will have time to bond with them and get used to her new role. Then introduce them as a group to the flock, while the chicks are still under mom's protection. She should wean them when they are around 6-7 weeks old.
this is the nest box and its about 3 feet off the floor.
