|Lorin King, released under CC 3.0|
We focus on how to read them, then how to convert one reading to the other. If a child barely has a grasp on Fahrenheit, it's really too much to ask her to convert to Celsius, not matter how much more sense it might make. Kelvin is just Celsius with a degree in pedantics.
A child can observe the triple point of water, at least two parts of it. (Water vapor is invisible--the fog you see is condensed water--droplets of liquid.) A child can tell when water is boiling.
Give a child a thermometer without numbers, without lines. Let her figure out why calibration matters. Otherwise a thermometer is just another talisman in a republic that cannot survive magical thinking.