'Blue Herring' as a Validating Form of Argument?

The Red Herring. It's agreed to be a fallacy of relevance where you distract from the point being made. However, in a complex discussion, sometimes grand narratives or concepts are discussed, which involve one or more vague/unexplored thoughts. Asking for elaboration could be seen as advancing the topic (rather than changing it), though it leaves you open to the charge of 'red herring'.

What do you think about this scenario?

Heather: The Child-Free movement is irrational.

Kimberly: How so?

Heather: Not having children is selfish.

Kimberly: So, *sips tea*, what do you consider to be selfish?

This argument applies to both pro-natalists and anti-natalists who both try to argue that not having children, or having them in any quantity, is a selfish act that ultimately harms others (either in the form of society breaking down, or the environment). Heather argues from the pro-natalist point of view, but fails to elucidate her point, so Kimberly shoots a 'blue herring' to ask her to expand her POV. Unlike a red herring, a blue herring stays true to the original topic.

However, the resulting conversation could result in a significant deviation from its starting point - Heather's original comment...

Heather: Well, selfishness is effectively acting exclusively in self-interest. Not having kids priorities your desire for freedom over the establishment of a solid, virtuous family. It is a selfish endeavor.

Kimberly: But according to this 'logic' brushing your teeth is selfish, since it's exclusively in your self-interest as well. After all, if you don't, you're the only one who suffers - they're your teeth. Instead, a better definition would be 'self-interest to the detriment of others.'

Heather: You're proving too much, because that's exactly what not having kids does. You only think of yourself and your own wants, seeing the lives of others as an inconvenience or burden. If our mothers thought like this we wouldn't have been born.

Kimberly: Heather, it's self-interest, but how is it to the detriment of others? Who are you harming by not having children?

As you can see, the conversation has veered into a long, philosophical discussion of selfishness. This could be considered a distraction, but I would disagree - challenging someone on a vague point is fruitful. 

Thoughts?

(For the record, I don't agree with Heather. I think women should make their own decisions on whether to have children - or even parent some at all).