The narrowest pa of trachea in a newborn is at the level of:

Neonates and infants have, compared with older children and adults, a propoionately larger head and tongue, narrower nasal passages, an anterior and cephalad larynx (the glottis is at a veebral level of C4 versus C6 in adults), a longer epiglottis, and a shoer trachea and neck. These anatomic features make neonates and young infants obligate nasal breathers until about 5 months of age. The cricoid cailage is the narrowest point of the airway in children younger than 5 years of age; in adults, the narrowest point is the glottis. One millimeter of mucosal edema will have a propoionately greater effect on gas flow in children because of their smaller tracheal diameters. Ref: Butterwoh IV J.F., Butterwoh IV J.F., Mackey D.C., Wasnick J.D., Mackey D.C., Wasnick J.D. (2013). Chapter 42. Pediatric Anesthesia. In J.F. Butterwoh IV, J.F. Butterwoh IV, D.C. Mackey, J.D. Wasnick, D.C. Mackey, J.D. Wasnick (Eds), Morgan & Mikhail's Clinical Anesthesiology, 5e.