Cocaine, breast milk
is metabolized to benzoylecgonine, the marker for Cocaine ingestion.
Your breast milk would contain this metabolite, not Cocaine directly. There are other products also. Your baby does not have the enzymes to inactivate cocaine, so is more sensitive to it than you. These products will be detectable in your breast milk 12 hours after use, but are often undetectable by 36 hours after.