How many eggs is she sitting on? Over the next few days, they should continue to hatch and you'll have a bunch of grandchicks
. The babies can live off their reserves (they eat the last of the egg before they hatch) for a couple days. The chick probably won't venture too far from her mama for at least a couple days. Once the hen gets a nestfull of babies, though, they'll be running everywhere. Watch her closely for a couple days. At this point if you try to move her, she'll probably abandon the rest of the eggs to take care of her one chick.
Once the rest of the eggs hatch (or all that are going to), OR the baby gets adventurous, she will come out of the nest. And 18 inches is a bit of a fall for a little one.
The only time we've had trouble with the older chickens picking on the babies (and yes, kill some of them) was when they were penned in a run. If your chickens are free range, then the mama will keep them away and/or protect them. If they're all penned together in a run, though, I'd separate them out into their own little run area.
Let us know how it goes. And congrats again