My daughter also has fine hair and grew out her bangs around the same age. It's good to do it now and get it over with!
I'm sure you've discovered that barrettes don't stay put in fine hair. The best solution we found was hair bands. The molded plastic ones with little teeth at Target worked best for her. Not every kind works for every shape head though. Some "popped up" on my daughter. We tried the elastic style and they were too big for her head shape and it didn't really work to make them smaller. Maybe they come in smaller sizes now though. If you sew, you might also look into making some scrunchie style hairbands that will fit her head.
She went to a party at Libby Lu or one of those places and they parted her hair into rows and twisted each group of hair tightly and placed those little clips at the crown of her head. That looked really cute and stayed for the afternoon, but I could never recreate it successfully myself. Maybe with gel it would work?