The best way to do it, I think, would be to forgo the hyphens completely and go with: I am a psychologist who works with children and adults. Anything else is awkward, ambiguous, or both. If it's necessary to emphasize that this individual works with children and adults (i.e., this isn't just introductory information), you could add in a 'both': I am a psychologist who works with both children ...
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