Is there sin upon one who was forced into a sin in his childhood that resulted in physical harm?

What you mentioned happened to you when you were not yet accountable, so there is no sin upon you; due to the hadith: "The pen has been lifted from three: from the child until he reaches puberty..." You should forget this matter and remain steadfast in obedience to Allah. As for the tearing of the anus, you should consult specialized doctors and take their prescribed treatment, and there is no harm in exposing the 'awrah if examination or treatment necessitates it.