Consider an aquarium
They'll learn more from it than nearly anything else. And what's more, if you get them interested, they'll *want* to learn. It's one of the cheapest forms of education you'll ever find.
A tank has so much to offer.
A kitty or a puppy, if the child tires of it, mommy and daddy will just pick up the slack. A tank, unless mom and dad have *had* fish, the child *must* learn the care or the creatures will die. (if mom and dad have had fish, they know well enough to let their child figure it out on his/her own.)
A tank teaches chemistry. You've got to have that bloody water right, or you'll lose everything. You *will* learn the nitrogen cycle. You'll learn all about PH. You've got to get it right. You *have* to. Life depends on it.
You'll learn all sorts of biology.
If you have livebearers, you can witness birth. And if you have too few hiding places for those fry, you'll also witness death. Livebearers eat their young.
You'll see death anyways. The longest-lived freshwater tropicals kept mostly live maybe five years max. And it's near impossible to go through a tank without goofing up. And then you get to witness the food chain and the lovely cannibalistic habits of most fish. And you'll learn about survival of the fittest and species interaction. After a while you *know* what fish to put with another, so as one doesn't end up just expensive munchies for the second.
You'll learn the power of creation. You have an entire universe at your fingertips to shape as you alone will. You will learn that, in some way, thou art god.
You will learn these lessons, and more. But one thing is sure.
You WILL learn.