Chibiusa goes back in time to obtain the Silver Crystal on the day of the Black Moon attack. Therefore, she meets her parents, including the future Neoqueen Serenity. Therefore, Neoqueen Serenity has advance knowledge of what her daughter is going to do and...just lets it happen? Just lets her kingdom get at least temporarily annihalated? Lets her daughter suffer that massive burden of guilt? However, if she does take the practical step of hiding the Crystal in a place Chibusa can't find it, Chibiusa never finds it, which means Neoqueen Serenity never meets her in the past, which means she wouldn't hide it so she'd go back in the past anyway and now my head hurts and if they were going to give Usagi and Mamoru a kid they should've just done a time skip because my brain only works in straight lines.