Parents listening to the singing of their young daughters

 Igros Moshe (O.C. 01:026):Listening to a young girl singing. This that you write that it is impossible to protest that parents should not come to listen to their children singing without causing great controversy, therefore you want to know my view regarding what the halacha regarding listening to children singing?  Beis Shmuel( E.H. 21.4) explicitly states that the voice of an unmarried woman is permitted except during prayer and the Magen Avraham (E.H. 75.6) states that only the singing of a married woman is always prohibited.  This that an unmarried female is permitted is only if she is not a nida.  This is stated by the Pre Magadim and it seems that nida is viewed as erva. The Be’er Heitiv (E.H. 21.4) cites the Be’er Sheva saying that unmarried females or a widow is prohibited and only the voice of a man’s wife is permitted except during prayer. That seems to mean that even an unmarried female who is not a nida and thus disagrees with the Beis Shmuel. Thus the halacha is in accord with  the  greater number of those that permit and furthermore the Be’er Heitiv does not note that the Be’er Sheva disagrees which indicates he has no concern that that view is halacha.In addition the Mishna Bereura (75.17) also permits an unmarried female who is not a nida when it is not during Shema. What needs to be a point of concern is that which the Mishna Berura writes that the singing of an unmarried girl is permitted as long as there is no intent for pleasure so that it doesn’t lead to lustful thoughts. So possibly there are some who come with the intent to listen and have lustful thoughts which is prohibited. But it seems that the Misha Berura means someone who listens for lustful rasons.and we can assume here also that people are not coming to listen for lustful reasons and we don’t need to be concerned that lustful thoughts will be produced by listening to girls singing. Thus the prohibition only applies to females that are prohibited to him inherently to get any pleasure from them which excludes unmarried girls who are not nida. So even if there is concern that listening will result in lustful thoughts, this is relevant only for older girls who might become nida and not young girls. In addition there is normally no lust for sex with children and consequently looking or listening to children does not produce lustful thoughts even when done for the sake of pleasure. This is true even for girls older than 9 years. In conclusion you asked for the actual halacha and that is that it is permissible for girls singing as long as they are not more than 11 ears old. More than 11, even if it is known that they are not nida and according to the straight halacha it is possible to be lenient as I have explained, nevertheless you should be strict since it is possible for some to be nida, If there is no pressing need you should always prohibit because in these matters all who take the strict view are called holy.