@LuluBlakey1
DD has asked for 'a real fairy to have as a pet'.
DS1 asked for 'a shark's tooth' - he's not getting one unless we find one on a beach.
Presumably you have explained to your DD that fairies are free living beings that don't need or want humans to look after them, and that even if it was possible to catch one (which of course it isn't as they can fly incredibly quickly, and can do magic, which includes invisibility spells. Which of course also means that actually getting to see a fairy is very, very difficult - I have spent hours standing stock still, barely breathing, because I didn't want a sudden movement of mine to scare any away, and I still didn't get to see one properly. But sometimes on a woodland walk, if you keep very quiet, and are very, very lucky, you can glimpse one as a very quick flicker from the corner of your eye, but sadly when you turn your head to see her or him properly, the movement scares them into becoming invisible again), it wouldn't be right to do so.
I am sure that you will have gone on to explain that there are only a few animals that it is kind and fair to keep as pets, ones that have become dependant on us. The best example of which are of course domesticated dogs. Except in countries that still have completely wild dogs, like Africa and Australia, most dogs don't live naturally in canine packs any more. They have become dependant on us for not only their food and shelter, but particularly for our love and care, and they usually reciprocate by making our lives a lot more special too. Personally I would struggle to explain how it would be kinder to keep creatures like birds, hamsters, fish etc as pets because I think that they would usually be much better off with having the freedom of their natural habitats. Of course if someone has the space, the knowledge, and the dedication to look after animals that would have a much lower survival rate and/or face extinction in their natural habitats, then having them as pets can be beneficial on both sides. You obviously will have your own views on what animals it is kind to keep as pets, and will hopefully be able to pass that on to both your DC.
As others have said, a sharks tooth can be bought very cheaply, and I am sure some of them will have been ethically sourced.