I'm glad to hear mom is taking this seriously. So many would think "it's just an animal". I'm glad your mom doesn't feel that way.
Again, I work with children ages 5 to 10. And I raised two girls of my own. Who by age 6 felt deeply for animals of all kinds. Don't make the mistake of thinking she is too young to understand what she did.
What's disturbing to me is that she "shrugged and smiled" when you asked her where he was. Looks like she knew it was morally wrong and did it anyways. This could be a red flag for worse things to come. Animal abuse at an early stage could be signs of schizophrenia which is disorder most serial killers suffer from. Sorry if I'm being grim and I'm not trying to over analyze the situation. Just giving you my input as more things for you to think about.
Ok, I think it's time to lay off a bit. It's not up to us to determine the correct punishment for one of our member's relatives. Let's all just give a big sigh of relief that the tortoise was rescued before the trash man came and picked up, and leave the discipline to the family.
When I made my comment, I was joking. I would NEVER do such a thing.
We're gonna' make Izzy wish she never posted that.
Certainly, your mother and you should provide some education on this subject. Wouldn't beat it to death with her though, as attention may fuel the fire. She will probably be embarrassed about this later when she matures. In the meantime, you of course realize, not to leave her alone with the little tort.