If the goal is to develop the top speed necessary to compete at the highest international levels in the 100m, then training for the 400m (especially at her age) is likely to have a negative impact. Running 400m now and again at a meet is probably ok, but all of that speed endurance work will reduce her ability to develop the top end of her speed potential.
For your daughter, if she loves the 400m, then by all means have her train for it and run it. But be aware that this will make it unlikely for her to be able to compete in the 100m long term.