Very cute idea. I'd get her something that she'll want to re-wear! If she's a sunflower fan, that's awesome. When I was a flower girl in my aunt's wedding at age 6, she got me a dragonfly hair clip because she knew how obsessed I was with them. The rhinestone color matched the bridesmaid dresses, but I only just now realized that, haha.
When you get gifts for bridal party members, littles included, it's definitely a great idea to shop for them like it's their birthday!