Alaska Pics (pair)
I've taught about 300 kids how to play kalimba, and about a quarter of them ended up using the Alaska Pics to prevent thumb pain.
You probably want to get XL sized piks - read on.
The Alaska Pic is designed to fit on the fingers of guitar players, so normal people like me need an XL size. If you have extra large thumbs, you might be out of luck. Medium size fits young kids approx. 6-9; Large fits kids approx. 9-13. There is a SIZE CHART in the additional images. The Videos Tab includes demonstrations of the Alaska Pik.
It takes you a few minutes to get used to the feel of the Alaska Pics on your thumbs -- basically, your nails are suddenly a quarter inch longer, so you have to recalibrate your hand positioning. But these pics do permit full thumb dexterity, and your thumbs won't be hurting (unless you order a size too small).
The price of $3.50 covers two pics.
Remember, you probably want to get XL sized piks, or possibly L if you have really small hands.