We are beginning siman 3. We have learned that there are two types of ervah, the primary ervah, and ervah which Chazal prohibited because they were concerned it would come to hirhur and distraction. As it pertains to davening and learning, Chazal did not forbid learning and davening in the presence of non-primary ervah because they were concerned about the hirhur aspect, but rather because of the distraction it causes to reciting devarim shebikedusha with proper intent. (This prohibition only applies to the non-primary ervah of a woman, not a man.)
In siman 1, the Chayei Adam had said that regarding katanim, the ervah of a girl is a problem from age three, and a boy from age nine. The Chayei Adam was discussing the primary ervah, which is why the ages given by Chazal are tied to the child’s ability to physically engage in biah.
Regarding the non-primary ervah, which Chazal prohibited due to distraction, the Chayei Adam understands that the issur applies from age 3 as well. The Mishnah Berurah understands this way as well. However, the Chazon Ish understands that since the reason for the issur is the concern of distraction, since people do not consider a young child to be attractive in a feminine way, the issur does not apply until the child is older. The Chazon Ish writes that it depends on the child and her physical maturity. Rav Chaim Kanievsky assumed that the approximate age this would apply is age seven. Until that age, one can recite devarim shebikedusha even if a girl’s non-primary ervah is uncovered.
The poskim seem to follow the Mishnah Berurah lechatchilla, but if one wishes to follow the Chazon Ish, they have what to rely on.
Rav Elyashiv discusses a community which wants to follow the Chazon Ish and allow girls into shul who are not covered until age seven. However, there is a minority of the community which wants to follow the Mishnah Berurah, and complains that they cannot daven in shul due to these girls. Rav Elyashiv paskens that if the majority of the community wishes to follow the Chazon Ish, the minority cannot compel them to follow the stricter opinion of the Mishnah Berurah.
We will see that these halachos apply for shema, tefillah, and divrei torah. Regarding tznius for the child herself (i.e., at which point she should be dressed in a way that these non-primary ervah are covered), some do apply the psak of the Chazon Ish, and understand that the chiyuv chinuch for tznius does not begin until approximately age seven. However, we learned in the halachos of chinuch (shiur 1263) that the point of chinuch is to prepare the child to learn how to act when they are older, and that this chiyuv kicks in once the child is old enough to understand what they are doing. If so, the parents would need to dress the child appropriately from a younger age not because there is a problem right now, but because it will be an issue when she gets older.
Those who are meikil understand that the chiyuv chinuch is to cover something which is ervah. If the body part does not yet have the issur of ervah, there is no chiyuv chinuch, because there is no issur. However, as we learned regarding the halachos of chinuch, the primary understanding is in accordance with the first opinion.
It is assur to recite devarim shebikedusha in the presence of primary ervah from when a boy is of age 9, and a girl is of age 3.
Regarding non-primary ervah, lechatchilla one should be machmir from age 3, but one can rely on the opinion that the issur does not apply until the girl has begun to show signs of feminine maturity, which is assumed to be approximately age 7.
The chiyuv chinuch for tznius may begin at an earlier age, and obviate the above leniency. Others are meikil until the age described above.