The classic affectionate metaphor
小棉袄 (xiǎo mián'ǎo, 'little cotton jacket') refers to a daughter — the idea is she keeps her parents warm. The phrase 'A daughter is the parents' little cotton jacket' (
女儿是父母的小棉袄, nǚ'ér shì fùmǔ de xiǎo mián'ǎo) is a fixed saying.