@fxtrtr - welcome to MobileRead.
AnemicOak has it spot on, a custom column is the best solution to this. I do exactly the same thing for sending books to my parents, I just use a yes/no column but if you wanted to keep track of multiple kids you could do something different. Or indeed just use a tag.
From a plugin perspective the closest in principle to what you describe is the Reading List plugin, which allows you to apply a tag/custom column value when a book is added to a list. If you don't intend to physically sync that reading list to a device (and are instead just using save to disk to send books or emailing for instance, you didnt state how you share with your kids), then the plugin won't add a huge amount of value to that workflow for you.
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