Does a landlord have the duty to find out if a child less than 6 years of age resides in an apartment in a multiple dwelling in New York?

Yes. The landlord is required to ascertain whether a child resides in a dwelling unit. They must make yearly inquiry and the forms which they must use are promulgated by the Department of Housing Preservation of the City of New York. Of course, the tenant has a duty to keep the landlord apprised so if a child begins to reside in the premises after the annual notice is answered, the landlord will be on notice under the law. The form used by a landlord and tenant can be obtained online at https://www1.nyc.gov/assets/doh/downloads/pdf/lead/lead-annual-notice.pdf