Take it to a place that packs and ships antiques to have it foamed and boxed up. If you know the maker of the harp they may be willing to sell you a shipping carton, too, or have some advice about their preferred way.
EDIT Oh, and take the weather into account. You can’t know for sure that the harp will be in any kind of climate control along the way, so you want to avoid times when it’s freezing or very hot.
I was assuming that your mother already has a harp that she wants to give to your daughter. If that’s not the case, any place you buy the harp should be able to arrange shipping. They do that all the time.