Talk to them about your game, making clear that what you have planned may include elements they find shocking
If you haven't started the game yet, or even are only a few sessions into it, you can still hold the Session 0 in a campaign-general way (that is to say, this relevant story might be far enough in the future that talking about it won't immediately give things away).
You can be clear about the types of ethically shocking content that may come up in the game without being very specific. When I do this I try to express generic things about the setting and tone. I introduced a campaign I'm currently running this way:
This is intended to be a dark campaign in which hope is not only fading from the world, but has largely already faded. The outcomes you want for specific situations might be impossible, even when victorious. If you want to try making things better, you can, and you can even succeed, but the game world itself will make that difficult and will try to grind the hope out of your characters even as they struggle.
Your characters may find themselves in situations where they have no good choices, or where choosing the lesser of two evils is barely an improvement over alternatives.
And ask your players for things they don't want to see, given the darker themes. In my most recent game, one of my players volunteered that he did not like violence against children (though they clarified that it's potentially OK as a plot element, they don't want to be forced to do it or see it happen "on camera").
Otherwise, if you are really concerned about the fit for this plot and the group (which may be wise, given that you don't know them so well), don't use that plot here. It's as simple as that. In my experience as a DM, plot ideas are never wasted simply by not being included in a current game or campaign; they're always available for future stories, and are often better for having more time to develop.
If you do use the plot, make sure your players have the information and agency to make ethical (or unethical) choices, and potentially opt out of this specific situation
This is the part where it's easy to slip up. Ethics require knowledge and agency, and because D&D is a game (with some very famous characteristics) it's common for players to make assumptions about the challenges being presented to them. That's a big part of why the newness of the group is an issue-- they don't know how you run games, and you don't know how they play them.
So the ethical dimensions of the plot need to be clearly presented in advance in order for your players to have the best possible chance of expressing their ethics in play. Crucially, the plot should not force or trick them into doing something bad like this. Just because you have sketched out a plot element in which the dryad uses children as human shields doesn't mean that that situation should actually arise. Especially if you choose not to have a Session 0 conversation (and I really, really encourage you to have that conversation), there should be off-ramps your players can use to avoid this situation.
Then, check in with your players often about the tone. If you find out one session before this fight that the group isn't OK with the situation you've designed, you still have time to change it. A DM's role is to make the game fun for the players, and part of that is making sure that they're OK with the game's content. That you wanted to maintain the surprise of a situation isn't your players' problem, and you should not make it so.
Absent a discussion covering this type of scenario (and, honestly, even if you have had that discussion), at a minimum I would suggest the following (roughly in this order):
Consider using other sorts of peril for the children to give the
dryad an edge in the fight
Obstacles are often important in combat encounters, but there are ways to use them without making the children into enemy combatants. If the dryad's goal is to use the children to distract the party, she could do that by having the children attack the PCs. But she could also do something like place them in immediate, conspicuous danger and force the players to use their turns to save them rather than fight the dryad. If they constantly wander towards a bonfire, the edge of a cliff, or a deep pool you can make it obvious that they are in danger unless the players intervene.
That's similarly helpful to the dryad (especially since you have control over her stats and the encounter), but it frees the players from having to make an active decision to hurt one of the victims. DMs have to live in narrative and mechanics, and as children are unlikely to be dangerous enemies their main mechanical purpose is to mess with the players' action economy. Anything which serves that mechanical purpose will do, and then you can fit it into the narrative.
The players should learn about children disappearing from wherever
the dryad kidnapped them, with some elements that establish the
children as characters
They're people and children, not faceless NPCs. The more they seem like the latter, the easier it is to view them as goblins or kobolds, and players rarely hesitate to kill, or feel bad about killing, most combat opponents. Give your players the chance to care about them, and they might!
Establish that the children were likely (or definitely) kidnapped,
as opposed to having chosen to go with the dryad
The more obvious it is that they are kidnapping victims, the more likely it is that your players will see them as victims to be protected instead of obstacles to be cut down.
Set up that the human shield-like behaviors are bizarrely unusual
for the children
If you drop some hints that an easily recognizable child isn't at all violent, then an action like attacking the party is a pretty good clue that something unusual is going on. This reinforces the idea that the kids aren't a mob to be put down.
Make absolutely certain that the combat is possible to win without
hurting any of the children, even if it may seem difficult to do so
Players tend to not like feeling railroaded, and if any of them have even a slight problem with hurting the NPC children it may be deeply unpleasant to feel like you set them up to do it without any alternatives. If the encounter design is even modestly off, it could feel as though you forced their characters to do something with which they deeply disagree. That's not epic or exciting.
The above list, in summary, is meant to make sure that your players have enough information to understand the ethics of the decisions they make and also that they have options to put their ethical decisions into practice. Without those elements they don't have any ethical choices to make, and that radically changes how events feel as they play out.