It's a reference to children.
Usually used in things like "Soon we'll hear the pitter-patter of little feet" to mean "We're having a baby."
You could also say "I hear the sound of little feet" to mean that the children of the household are awake and on the move. And probably hungry. And perhaps breaking something. Or coloring on the walls. Or giving the cat a bath in the toilet. Or... anyway, they are up and about.
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