`千金` vs. `女儿 (nǚ'ér)`: `女儿` is the neutral, everyday word for “daughter.” You use it in most situations. `千金` is a respectful, affectionate, and somewhat literary term. You wouldn't use `千金` on a government form, for instance.
Don't Call Your Daughter `千金` to Her Face: It's a term used when speaking about your daughter to others, not to her. Calling her “Hey, `qiānjīn`!” would be strange, much like an English-speaking father saying, “Hello, apple of my eye.” You would use her name or a nickname like `宝贝 (bǎobèi)`.
For Daughters Only: `千金` is exclusively for daughters. The male equivalent for “your esteemed son” is `令郎 (lìngláng)`. A more general, historical term for a nobleman's son is `公子 (gōngzǐ)`. Never refer to a boy as `千金`.
Context of `千金小姐` is Key: Be mindful that `千金小姐` can be either a neutral descriptor (“heiress”) or a critique (“spoiled brat”). Pay attention to the speaker's tone and the surrounding conversation to understand the intended meaning.