Above: Normal adult keyboard compared to childsize kids keyboard. Shown: Little Princess Pink & Purple Keyboard
One-handed adults do NOT need a special one handed keyboard.
I am a one handed typist, and I do 40 - 80 wpm on a normal keyboard.
They just need the One Hand Typing Manual to learn to type. See more about One Hand Typing Software and Manuals
Your child may be typing on a normal adult size keyboard, but they are NOT typing well. It is impossible for their little fingers and smaller hands. But they are training themselves to type poorly as a "hunt and pecker." A habit that is going to be very difficult to break as they get older. You would not ask a child to ride an adult bike. Don't ask them to type to learn to type correctly on an adult size keyboard. They needs a Kid's Keyboard.