how to make subject [people] face slimmer in Photoshop Elements?

. . . . The Perfect Portrait Guided Edit reduces the width of the entire photo.  The Enhance>Adjust Facial Features menu option can change only the width of the face.

I agee.

Also, have a look at the guided edit mode:

- special edits

- perfect portrait

This 'slimming' is needed for selfies which are taken too close from the face. With older PSE versions without the above features and if it's a full frame portrait, the tool to use is the Filter Camera distortion. The 'remove distortion' slider is used to correct barrel distortion which makes the nose less prominent in the pictures as well.

Yes.  Go to Enhance>Adjust Facial Features: