Children hold breath.
Meaning: This means that multiple young people are stopping themselves from breathing in order to avoid making noise. It's like saying, The children are holding their breath so that they won't make any noise and give away their hiding spot.
Choice of Words: Children is a plural noun that refers to multiple young people. Hold is a verb that means to keep or maintain something in a particular position. In this case, the word breath is used to describe the air that is inhaled and exhaled by a person. When you put them together, "hold breath" is used to describe the action of stopping oneself from breathing in order to avoid making noise.
Alternative Expressions
Related Expressions