You are looking for a "Culture Pass", the museum I volunteer at in Connecticut gives them to the local library and the terms are stated on the pass, it could be a BOGO or children free whatever the museum gives the museum. Some even have a up to value.

NYC libraries have them for NYC museums though I assume you don't live in NYC but if you work in NYC you can get a NYC Library card.
https://www.nypl.org/blog/2018/07/16/culturepass