I train Japanese Samurai, and use a wooden Katan for medium drills, and a live blade for slow drills.

When we spar, we use foam blades that are about 3 feet long, not including handle. I think most martial art stores will sell them. I have also seen shorter handled swords (I train and use an 12 inch handled Katana) just big enough for one hand, which would be baout what you would want for fencing. They are also available at martial arts stores.

I am not going to say there is no possibility of injury with a foam sword; I got bruises to prove it. But most children with little training will generate the force necessary for an injury.

If you can get that baseball bat swinging habit ended now, and get her to take small, low steps, your child will be miles above the rest when she starts formal training.

Good Luck