I would definitely bring some "Theresian" spiritual insight to the girls. Especially because of St. Therese's young age, I think that they will be able to relate to her [I know I can, and it is not because of my sanctity!] Make sure they realize that a vocation is a gift from God, a baptismal gift we receive from Him, one that, sometimes like many physical gifts, takes us a while to really appreciate.
Our vocation is the way God is calling us to love. That is the most important thing that girls could get out of any talk, since it leaves room for them to really find God and discover their vocation on their own without feeling like one state-of-life is better than another. While religious life is a higher calling, that doesn't make it
better. I think it is especially important to teach people that, or they could get a false sense of religious life, believing that religious are out to put on "holier-than-thou" airs; it's the beautiful unity of a human's body and blood to God while still on this earth. Possibly present the girls with a panel - a priest, a Sister, a married couple, a single person. Having a priest and a married man there might bring light to things the girls have never understood. I know for me that when I witnessed a talk with a panel, my knowledge and awareness of the priesthood definitely increased.
I would suggest watching this video for some ideas:
http://www.pbs.org/wnet/religionandethics/...1123/cover.htmlGod bless you and I will keep this in my prayers; I know that your talk will go well no matter what you say - you are the pencil with which God is using to write His words of Love in these young girls' hearts. <inspired by Mother Teresa