Does she have plenty of hay elsewhere that she can munch on instead of the hay in the nest? Rabbits only nurse once a day, at the most twice, so she probably won't feed them until tonight.
If she has plenty of hay in her cage that she can eat, and she is still going in the nest box, you may need to pull it out of there until this evening, so the kits don't get trampled. Then this evening put it in there to see if she will nurse them. If not, then you may need to pull it back out and try again later. If they haven't been fed by morning, when it's been more than 24 hours since birth, then you will probably need to step in.
I just want to warn you to be prepared for this not to work out. First time mothers often have issues knowing how to be good moms, and sometimes these litters may not survive. There are some things you can try if you need to intervene like holding her and carefully putting the babies to her belly to nurse, but they don't always work out.