Are severe tribulations from an early age evidence of sins committed by oneself, or could they be for raising one's rank, or an expiation for the sins of one's parents, or protection from a trial, and not necessarily due to the person's own sins?

A tribulation does not necessarily have to be a punishment for a sin. It may be for elevating one's rank and increasing good deeds. A Muslim's duty towards it is patience, submission, and acceptance of Allah's decree and destiny, along with supplication. As for a tribulation before puberty, it is not a punishment, as the pen has not yet recorded deeds, but rather for reasons known only to Allah the Almighty.