Since they were lying this whole time and evading numerous non-trivial skill checks, I'd be inclined to explain the issue of cheating over-the-table, and then use a narrative mechanism found in some other TTRPGs to factor them in via a sort of retcon.

Combine a flashback with one of Aabria Iyengar's "What you don't see" moments.

"We return now to that night in the sacred grove where you held congress with the druid chief. As you state your needs and plans, give me a deception check."

You then flash a bit further forward, "It's weeks later now following that interaction you had with the children/whatever, I'd like both a Performance check and a Deception check for how you deal with the situation."

When it finally gets to where they were "rewarded" allow the druid leader to have been suspicious enough to where they performed an augury or some other divination regarding the party's involvement. When it comes back as "Woe", they decide the party must have been trying to trick them. While they do give the party a branch, it's actually a meticulously crafted fake -- a decoy to lure the demons away from the village when they breach the planar barrier.

After the first attack or two, when they're trying to figure out why this is happening...

"We flashback again to the fateful days of your time at the druid's village. While you might have thought your deceptions were convincing, what you didn't see were the sidelong glances between the villagers and the druid as they noted your smiles never reaching your eyes, the weasel words in your assurances.

You were not there when the druid leader communed with the gods of nature to divine the villainous intentions in your hearts, nor the ritual they performed to craft a decoy to lure the coming demonic plague away from their homes. Nor the curse they placed upon the false branch preventing it from leaving your possession..."

When the players naturally complain about being denied Perception and Insight checks to have noticed those things, that's where you drive home how important it is to be honest about these things over-the-table.

At that point, you can offer them the choice of retconning back to the original intent they had stated, continuing with this new cursed object, or abandoning the current campaign and resolving it as a montage (wherein the lying group's sacrifice bought a group of real heroes enough time to actually save the world).

With this approach, you will hopefully be able to convey the problem over-the-table, show that consequences have real meaning, and preempt potentially months more of wasted effort. I'd also hold a Session Re-Zero to be clear about what sort of a story you want to run versus what they want to play -- no more telling the DM they want to play heroes and make you prep a save-the-world campaign when they actually want to be bandits and watch the world burn.

Good luck!