The Laws of Prayer (75)

1. In the blessing of “Hashiveinu” Chazal fixed the expression referring to Hashem as “Avinu” (our father) as in it we request for learning Torah, and we mention before Hashem that the father is obligated to teach Torah to his son, and we are also sons of Hashem and are requesting that He should bring us back to learning Torah. (Mishne Berurah 115)

2. In the blessing of “Selach Lanu” we also mention the expression “Avinu” as the verse says regarding the sinner (And he will return to Hashem and He will have pity on him!”, and we call Him our Father in order to arouse the mercy of Hashem to forgive us. (ibid)