Watch, watch watch her. When they're peeping and getting ready to hatch, just keep an eye on her, she won't move them till most, though possibly not all have hatched. Once they're almost all hatched, she may well push any unhatched eggs out from the nest and sort of 'concentrate' on the little peeps. Once they've all fluffed up, dried out and seem 'settled', she will have sort of nestled them in and gotten comfortable with them. Anywhere from 6 to 36 hours I'd say, and this is not a hard science, it sounds like you know this hen's temperment... I want to say at that point she's mostly 'bonded' with them, but I'm not sure chickens actually bond. But that's the safest time to move her and them. Get help to do this, even if it's someone down below to hold her in a dog crate so you can slip the chicks into it with her.. yes, she's going to freak out, but you move them all into a safe spot and all will be well.
If worse comes to worst, and she does 'boot' them out of the loft, chicks sort of *Poof* to the ground amazingly well. They can actually handle it quite well and I wouldn't freak out over it.
//edit to add// this is only what I would do. I've never had one nest quite that high, though I've had them up about 4.5 to 5 feet. I would put some extra loose hay or straw below the area just in case if possible, though that may not be practical. I have seen them poof down amazingly lightly, so I'd advise you not to panic!