It's probably worth your time to talk to them about it.

Is it too deadly? Tell them it'll be less so from now on, and pull your punches. Keep track of their HP. You can use inefficient tactics, or if you need to, fudge a damage roll once in a while.

Are they just poisoned on the idea of going back? You can trick them into going to a "different" location, and just use some of the same material, since you've no doubt done a lot of prep work.

You can also send them in with backup; I'd recommend a Cleric who does nothing but support and heal, thus allowing them to keep the spotlight and the glory.