I don't know if your DD plays the piano or not, but when I was a kid I always thought of middle C as zero, and anything to the right of that was a positive number and anything to the left of that was a negative number. I also thought of the white keys as whole numbers and the black keys as decimals or fractions (although looking at it now, obviously the black keys don't have an exact correlation to fractions or decimals). Anyway, it was just the way my mind made sense of negative numbers when I was a kid. I really like the sea-level and ground-level explanations others have mentioned.
