As always, please consult your rabbi for a practical ruling.
The Seridei Eish (2:8) defends the practice of women singing zemirot on Shabbat by citing the Sdei Chemed (Klalim, Maarechet Hakuf, 42) who quotes the Divrei Cheifetz who asserts that the Kol Isha prohibition does not apply to women singing Zemirot, because men do not derive pleasure from the woman’s voice in this context. In fact, the Pasuk (Shoftim 5:1) records that Devora the prophetess sang a song of praise to Hashem together with Barak the son of Avinoam.
Rav Mosheh Lichtenstein argues that the ruling of the Divrei Cheifetz and Seridei Eish permits listening to a woman singing at official Tzahal events that are formal and serious. He first bolsters the opinion of the Divrei Cheifetz and the Seridei Eish by demonstrating that Rambam, Rashba and Ra’avyah agree that the Kol Ishah prohibition is not absolute but rather emerges from concern lest it lead to sin. Thus, in a context that concern for sin is not relevant, the Kol Ishah does not apply.
It may be possible to extend R' Lichtenstein's reasoning to other formal and serious events, such as the one in the question, especially given that singing holy songs should not lead to concern for sin.
Rav Moshe Feinstein (Teshuvot Igrot Moshe O.C.1:26) and Rav Ovadia Yosef (Taharat Habayit 2:270) rule (based on the Mishna Berura 75:17) that in case of need, one may rely on the ruling that the prohibition of Kol Isha does not apply to girls who are not Niddot. Rav Moshe writes (in 1947) that one may assume that there is no question with girls below the age of eleven. Rav Moshe writes that men must be strict regarding girls older than the age of eleven, since there are girls who “nowadays” become Niddot at the age of eleven.
Source: http://www.tabc.org/kol-torah/article/index.aspx?pageaction=ViewSinglePublic&LinkID=712&ModuleID=43&StartDate=4/5/2013&NEWSPID=1 (alternate location: http://koltorah.org/ravj/The%20Parameters%20of%20Kol%20Isha.htm )