Is it obligatory for a girl to inform her fiancé about the loss of her virginity due to an incident before puberty, and what are the consequences of not informing him of this upon marriage?

What you have mentioned is not a justification for shunning marriage. Rather, accept anyone whose religious commitment and character you are pleased with, and do not pay attention to the whispers of Satan. The mere presence of blood is not proof that virginity has been lost. Even if you were certain that it was lost, you are not obligated to disclose that. The Permanent Committee for Scholarly Research and Ifta has issued a fatwa (definition) permitting concealment in such cases, and if the husband asks her after consummation, she should tell him the truth. Our advice to you is not to turn away from marriage, and to have a good opinion of Allah.