Aww, how cute is that? She could just carry a flower ball or wave a ribbon wand. They have small wrist corsages with ribbons and I've seen very small and simple bouquets called posies that you could add ribbon to and she could wave that like a princess scepter. You could also have someone escort her down the aisle either walking or carrying her to make sure she doesn't eat the petals. It's going to be super adorable either way!
Im actually having two 'flower girls' and one will hold this sign I made. I've seen a ton of different signs that can be done too, saying different things. You could also have her hold your train, if you have one. And I saw something somewhere where the little girl twirled a baton with pretty fringe on the end that matched the wedding colors.
I'm just having my 2 year old hold a fake oversized rose that she chose herself. She's super excited for it!
Also, I'm having her cousin, also 2 years old, hold the ring pillow (I sincerely doubt our actual rings will be on it), and her older cousin/his half sister, who is a tween, will hold their hands and escort them down the aisle.
My flower girl will be roughly same age (2 1/2) as princess Charlotte when she was a flower girl for Pippa Middleton. So, I decided to copy that. I found it by Googling 2 year old flower girls. It was a "omg, that's so perfect!" moment.
It's a heart shaped wreath of baby's breath on a Valentine's day wedding.
Other alternatives include: signs bubbles, small bouquet of anything, a teddy bear, or anything you think she can carry.