Temperature is just how fast the molecules are vibrating.
Absolute 0 exists because eventually they slow to a stop, and they cant vibrate slower than that
Except that ice is one of the few known exceptions, where the solid form of a liquid is actually less dense than the fluid form, but I guess that would be a detail you could likely skip talking to a 6-year-old. Unless they're smart enough to know how density works.
Ice > water > vapour
Boom, done, 3 year old satisfyingly taught.
Absolute 0 exists because eventually they slow to a stop, and they cant vibrate slower than that