If kids shouldn’t suffer for the sins of their grandparents then why should other kids suffer for the sins committed against their grandparents?

You can’t starve one man while feeding another for years, then give them both equal food for a few months and assume they’ll be even. You by necessity have to give the starving man more, and regardless of the fed man’s complaints it isn’t unfair for him to get less - he’s literally been getting more the whole time and is perfectly healthy.