There are tons of fairy garden kits for kids on Amazon and you can get a few at craft stores, but they are really lacking imo. I honestly don’t love the kit option, haha, I prefer to DIY my fairy gardens (but you knew that)..
Choose almost anything as a fairy garden container. I have seen some up cycle their old water tables into fairy gardens for their kids, or you can use a flower pot you have laying around!
If it’s indoor, I would suggest going super simple with some fake plants and a base of craft moss and stones – or plain old GRASS. For an outdoor garden, be sure to provide MORE drainage than you would normally think necessary.
I said it before – haha, this is for them and not for you. Ask them questions and encourage them to figure out where they want to place the houses, furniture and fairies!