The DSi's camera is probably the lowest-quality camera device you can buy today that can still counts as a camera. But, it's still a camera, and your kids might enjoy making their photos slightly more interesting than the default shots.
Thrustmaster's $20 kit offers variable color filters, a macro lens, a telephoto lens and a wide angle lens. It's great for illustrating the fact that lenses change the way a camera works, since it's not quite something that clicks solidly in your brain until you see it firsthand. But it's only $20, so it's not too much to waste when your kid loses all the pieces. [Thrustmaster via Ubergizmo via Crunchgear]