When helping a child deal with a fear such as fear of fire truck siren sounds, it is important to identify the cause of the fear and acknowledge that it is real for the child. Maintain a calm demeanor, and comfort the child with warmth and compassion.
Encourage the child to take slow, deep breaths. This helps reduce the body's physical reaction to stress. Offer physical support by giving the child a hug or letting him sit on your lap. When the child is in a more relaxed state, try to explain the noise to him so that he has a better understanding of the situation.
While a child should never be forced to confront his fear head-on, you can make the object of the child's fear less scary by playing with a toy fire truck together or asking him if he can make a fire truck noise with his voice. Framing the loud sound of a siren in a fun, positive context may help desensitize the child to his fear.
If you suspect that there is a deeper cause for the child's reaction to the loud sound of a fire truck siren, do your best to address the underlying cause, and seek professional counseling if necessary.