Suppose that I am talking to my friend about how to cheer up children, so which is correct?
When I think a bit more, both are correct: on their face means I am talking about it generally, in 'on their faces' I mean I am talking over children in front of me. Any suggestions?
a) Seeing their best friends would put smiles on their face.
b) Seeing their best friends would put smiles on their faces.