What is the origin of the idiom "knee-high to a grasshopper" and when to use it?

The idiom "knee-high to a grasshopper" is believed to have originated in the 19th century, and it is used to describe someone who is very young or small in stature. The phrase suggests that the person is as small as a grasshopper and barely reaches up to the knees of an adult. It is often used in a nostalgic or affectionate way to refer to someone's early childhood years or to describe a young child.