Usage patterns
にこやかな顔a smiling, cheerful face
She always greets customers with a smiling face.
にこやかに話すto speak in a cheerful, smiling manner
The teacher spoke with the students in a cheerful, smiling manner.
にこやかに応対するto respond/deal with someone in a warm, smiling way
The receptionist responded to every question in a warm, smiling manner.
にこやかに迎えるto welcome someone with a smile
The clerk welcomed the customer with a smile.