Stand together at a mirror where you can both see your faces.
Make one clear expression — happy, sad, surprised, cross — and name it. Hold it long enough for your child to study it.
Invite them to copy. Then let them choose an expression and you copy theirs. Finish on a happy face; the last one tends to linger in the mood of the room.
Copying an expression is not just imitation — the facial movement itself feeds back into how the emotion is recognised. Naming it at the same moment attaches a word, which is what later lets a child say 'I am cross' instead of hitting.