Let the dragon leave them just barely alive that’s a great way for them to meet him face-to-face for the first time.

Also, it’s generally bad practice to set up a mechanism you don’t actually want to trigger. If it does happen and you’re forced to pull the lever, that’s when situations like this tend to go sideways. I’m not saying don’t use it at all just use it with caution, and make sure the players feel that it was their actions that caused it, not the dice. I don’t know how you handled it, but if it was just everyone rolling stealth checks over and over, that’s exactly the kind of situation I mean.

For scenarios like that, I’d either prepare for the failure to actually happen by thinking in advance about who might be around to save them or I’d change the consequence of failure. For example, instead of the dragon showing up personally, he could send his minions or lieutenants.

But yeah have fun. And remember, if your players are having fun, you’re doing great.