I think it would really depend on the personality and disposition of the yearling. If he is quiet and lazy, your daughter might have no problems at all with him, if he's more spirited, he may be a handful for her.
Even if he is easy to work with, however, a seasoned older show gelding (or even mare) would probably be your best bet, just because an experienced will already be trained, which is a bonus, because your daughter, as a beginner, will find it a lot easier if the horse she is showing is not also a beginner.
Additionally, your daughter would be able to show a senior horse in more classes. She would only be able to show the yearling in Halter, Color, Liberty and Costume, whereas if she got an older horse she could show in those classes plus Driving, Obstacle and Hunter.
Good Luck!