Those doors usually take a lot of strength to open, but if you are worried you should be able to get some kind of device to lock it.
Is she able to figure out locks yet? if so, I would try to get something that fits in the top of the door to keep it from opening.
Another concern is making sure the balcony furniture is set up in a way that she cannot climb upon it.
I have never heard of a child accidentally ( or otherwise ) falling off of a balcony, but I would never take anything for granted, either.