For a fairly casual question I'd see it more like an ice breaker or a way to build some rapport, but it's down to how you ask the question as well. Letting people know in advance (as the parent suggested) rather than dropping it on the fly is a good enough accommodation for people who would be anxious about that.

I find interviews quite tense and I generally dislike having them (on both sides of the table), but throwing in a few disarming questions here and there throughout really takes the edge off. A conversational, or more cerebral, approach like that can suit some people far better than live code tests or quick-fire Q&A.