It happened in New Cassel on Thursday at 9:20 p.m.
A 28-year-old woman was walking along Tennyson Avenue with her one-year-old child in a stroller.
The first suspect blocked the woman's path, while the second suspect lifted up the stroller and demanded the woman's purse while holding a knife to the baby.
The first man then grabbed her purse and they ran away from the scene.
The woman and her baby were not injured.
Detectives are requesting anyone with information regarding this crime to contact the Nassau County Crime Stoppers at 1-800-244-TIPS.
All calls are confidential and all callers will remain anonymous.