How do doctors know what prescription is needed for baby eyeglasses, if babies can't read an eye chart?

Refraction: Refraction is the measurement of how much the eye bends light. Refraction can be performed subjecively - the doctor shows the patient an image through progressively stronger lenses until the patient sees clearly - or objectively, by measuring how reflected light from the eye is bent and using lenses to "straighten" the bend without the patient giving his/her input. This method is used for children.