Your grandneice’s mom sounds like a real peach!
From your description, I imagine you are pulling the front part of her hair (from the ears up to the crown) into a ponytail. And then the rest of her hair will be down. Right. Like half-up?
For the ponytail part, most stores will carry a variety of sizes of ponytail holders. You are going to want the smallest size that you are able to manipulate with your hands. And try to find ones without any metal in the loop. They are called ouchless, because hair doesn’t get stuck and yanked out in the metal bit… But you can make these ones even tighter. Goody’s is an excellent brand, and is sold at most pharmacies and Giants. Some stores even carry what look like tiny, clear plastic ponytail holders. These would be perfect, BUT they take practice using!!!
Do the half ponytail part first. Use a slightly wet hairbrush with lots of fine bristles, smoothing the hair back. Knot the ponytail holder as close to her scalp as possible. Once in the ponytail, spray the pulled back part lightly with hairspray, and using the back of the brush (flat part) smooth her hair away from her face, towards the ponytail holder.
For the curling part, spray each section of hair lightly with hairspray, and let dry for a second, before curling with a screaming hot curling iron. Curl the hair up to about an inch away from her head.
I highly recommend using an aerosol-nozel hairspray. Not a hairspray with a manual/mist spray, like a windex bottle. An aerosol hairspray will release smaller droplets of hairspray, and will cover her fine hair without saturating it. A windex-like hairspray bottle will release fat droplets of hairspray, that will likely saturate her hair.
Good luck and have fun!
ps. Please take this with a grain of salt… I am an amatuer hair stylist/make-up artist… i.e., I have done hair and make-up for a number of weddings and other special events, but have no real training.