You say 'hot water is heavier than cold water'. What do you mean by that? Do you mean per unit volume? Or do you mean per molecule? Or what?
I am guessing you mean per molecule, based on the idea that each molecule has more energy in hot water and energy is equivalent to mass. To a little girl though it's going to seem more intuitive to think by volume, in which case the hot water is less heavy. Also, I don't know if heat adds weight, even if it equivalent to mass.
If you're going by energy is equivalent to mass, the only way I can imagine trying to explain that would to be to tell the little girl that the warmth has weight.
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