How can I check if a new group is OK with a plotline from an ethical standpoint without spoiling the plot?

TL;DR: I am unsure if the plotline I planned is "too dark" for a new group, and would like to discuss the level of grit with them without spoiling the plotline. How can I do this?

I am planning a campaign where an evil dryad witch has kidnapped a bunch of children and is planning to turn them into trees which she can sell to the local sawmill. My idea is for there to be a grove where the kids are kept prisoner, where the heroes (have the option to) fight her and save the children.

The next idea is that she can mind control the children, using them as meat shields.

I feel like this could be a pretty epic moment if they manage to keep the children safe/heal them from the mind control and slay the witch. However, I’m worried that this triumphant moment will turn sour if they mindlessly hack and slash at the mind controlled children instead of saving them all, and realise only afterward what they have done.

The problem with all this is that I am planning this adventure for a new group – I have neither played nor DM:d with them previously (although I know some of them personally). The experience level ranges from near-beginner to intermediate. Since I am a bit concerned that the plot I planned could be considered "too dark", I would like to bring up the topic of what we consider in-bounds (as a group) in advance of the session. How can I start this discussion without spoiling the plot specifically?