Until what age may girls be present in the men’s side of the shul?
Some write that one is to abstain girls from being present in the men’s section beginning from age three and onwards, even if they are dressed modestly. The Rebbe however writes that the prevalent custom [Maaseh Rav], based on the consensus of Gedolei Yisrael, is to permit young girls to come to shul with their fathers and join them in the men’s section [so long as they are dressed modestly]. This applies even if the girls are past the age of three, until they reach the age of Chinuch and understanding regarding this matter. From age three, it is forbidden for an immodestly dressed girl to be present in the men’s section.
 Piskeiy Teshuvos 75/8 in citation of Teshuvah Meahava 2/229
 Sefer Hasichos 5749 p. 5 footnote 25
 See Maharahm Brisk 2/70 that even though in his opinion it is permitted to Daven in their presence, nevertheless, they are not to be brought to Shul when dressed immodestly.