One Medrashic source explains the appearance of the Cherubs that they were like children, based on the verse "כי נער ישראל" (Hosea 11,1). Moshe explains to Yehoshua (when changing the leadership) that the Jewish people were like children. Do not be too particular about their actions, rather excuse them like a parent condones the actions of their children, for this is what G-d does. Thus, the face of the Cherubs reflected a child’s face in order to allude to this relationship between G-d and the Jews. This allusion is a little difficult to relate to the time of the Bais HaMikdash, when the Jewish people were no longer immature in their relationship with G-d.
Another source (Yuma 54a) explains that in the time that the Jews went up at the festivals to the Bais HaMikdash, they would open the curtains to the Holy of Holies. And the people would see the Cherubs embracing and they would say to the Jews this is how G-d loves you.
It is explained that one Cherub would be the face of young girl and the other the face of a young boy displaying the nature of young love. Rabbi Bachaye attempts to explain that the word שני expresses that the two items were identical, but the word שנים (like here) does not imply identical. However, Rabbi Epstein points out that many places (including the daily Tamid offering) uses the word שנים while also implies the concept of identical items.
Rather, Rabbi Epstein says the faces were identical. And the embrace of the two Cherubs expressed a sincere hug of love, like that exists between a male and a female.
