While the sarcasm may be the most tempting answer to this DCM, I've found that if I communicate more openly and effectively, I'm more likely to get the response I want from parents. Although, there are certainly times when I want to say "here's your sign" and walk away.
Instead of your original scenario, perhaps things would work out better if you tried something like this:
DCM: So what are you having for lunch today?
Caregiver: (describes lunch)
DCM: They eat that? (With a disgusted look)
Caregiver: Yes, the children enjoy it very much.
DCM: Oh I don't think I would like that.
Caregiver: That's too bad. Have you ever tried (lunch food)?
DCM: No, I never really have time to cook/we usually eat out/I'm a terrible cook (whatever it is she says)
Caregiver: That's disappointing. Maybe you could join us for lunch sometime. Your son loves (whatever it is for lunch)
When you listen to her words with empathy (and I mean Listen not hear) and respond to the mother's needs, you may find that she's not trying to be difficult at all.
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