I've filed for my husband (CR1 spousal visa), and I'm a green card holder. Will his five-year-old daughter qualify for derivative beneficiary?
You will have to file another petition for the child. You will need to file a new I-130 and all the same documents that you filed for your husband, but you can petition for the child. Be forewarned that when a green card holder petitions for a spouse or a step-child, the spousal visa processing time takes substantially longer than if you were a US citizen petitioning for a spouse or step-child. A normal spousal visa timeline is generally under a year. But a spousal visa for an LPR, or legal permanent resident, can take up to two or three years.
If you are eligible to naturalize and become a US citizen, it may save you significant time to naturalize first, and then petition as a US citizen.
