Will Allah forgive a sin committed in childhood, and do I get intrusive thoughts that I am ugly and people will talk about me once my secret is exposed?

The child who has not reached twelve years of age is not held accountable, so misdeeds are not recorded against him, especially if he was ignorant that the act was a sin. Do not despair of God's mercy, for despairing of His mercy is a characteristic of disbelievers. If you had reached puberty when you committed the sin, then the one who repents from a sin is like one who has no sin. If some people became aware of the matter, this is a trial that requires patience, and thank God that it did not spread further. Remember that regret over a sin is repentance, so invest that in turning to God and supplicating for forgiveness, well-being, and concealment in this world and the hereafter.