Is it obligatory for a person to strive to erase the effect of a sin committed before puberty, such as issuing a fatwa without knowledge, and does the same ruling apply to backbiting and calumny committed before puberty?

If a person committed a sin in their childhood, they are not sinful, according to the hadith (definition) about the pen being lifted from three: one of them is "from the child until they reach puberty." As for the effects of that sin: if it concerns them personally, they must repent. If it concerns others, such as informing them of an incorrect religious ruling that people acted upon, then they must correct the error if they are able. Similarly, regarding backbiting and tale-bearing, there is no sin upon them before puberty, but if it resulted in ongoing injustice, they must remove it if they are able.