What's the best treatment to get rid of stridor?

See a doctor. Stridor is usually caused by swelling of the vocal cords or the nearby tissues of the airway and larynx. This can be dangerous or deadly if it triggers a laryngospasm. Prednisone or some other steroid treatment is the most effective to get the swelling down. In urgent or emergent cases Epinephrine in a nebulizer can be used as well.
If mild. Croup is usually mild illness managed at home. Help with cool m​ist, placing child near vaporizerI if you do not have a cool mist vaporizer, even a simple humidifier in child’s bedroom may be helpful. Seek ER if cool mist not clearing up stridor in 15 minutes, repeated episodes of stridor or trouble breathing child’s chest or stomach retracting, bluish lips, lethargy, drooling, neck pain, stiffness.
Race mic epi. Racemic Epinephrine nebulizer, possibly with addition of steroids.