A very famous improv comedy game is 'Yes and...'. The first person gives a statement and the second adds something with 'Yes and..' Then it's the turn to the first person and so on. It will be clear by seeing this fragment with kids who plan a party.
To be honest, I don't know if I can really use it in the restaurant. Maybe when there's a lot of time and with very open kids. I might try it by starting myself saying 'Yes and..' when a child tells a story and maybe the kid will pick it up.
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Ingrid Bergman gives a virtuoso solo performance in this Jean Cocteau one-act called The Human Voice. The plot of this hour-long piece is a simple one: a woman, devastated after her lover leaves her for someone else, speaks to him on the phone one last time. (in the theatre and I suppose also here in this fragment, there's nobody on the phone of course)
You can talk on an imaginary phone (use your POS card!) with an imaginary person. Also when a kid plays with something (that looks) like a telephone, you can play this improvisation. A month ago, I talked with a Romanian kid. She understood nothing (I talked Italian) and I understood nothing! But we both (and everybody around us) had fun!