Rules
A far better way would be to create a mail folder for the emails, then create a rule that moves them to the folder and marks them as read. That way you will not be bothered by the emails and you can honestly say that you have received them. If you don't want even want to do that, then create a rule that matches her email address and move directly to junk, marked as read. I'd not block your nieces emails, pity to hurt an 9 year olds feeling, they are too young to know better.
Besides it wouldn't hurt to keep them, and read a few before deletion, as who knows you may have an invitation or a specific request for a gift.
Rules are the way to go.