Hiya!
New players? Well, nothing teaches better than thinking you're cleaver and then proceeding to get your butt handed to you because you didn't think things through as much as you thought you did.
Don't deprive your players of this learning opportunity. Don't teach them that no matter what they do, or don't do, you will 'protect their PC's' from death. Don't. Just...don't. You will be doing FAR more harm than good, imnsho.
As they are new players, I would probably let them get as far as to see the dragon, roll to see if the dragon is asleep (I use the BECMI or 1e rules for dragons being able to speak or cast spells). Then I'd use my DM warning voice and say something like "Ok, are you sure you want to do this? It looks REALLY big and it is sleeping on a pile of both treasure and bones...mostly the later". If the players then decide that they can "take it", ok. Roll initiative. I would open up with the most devastating effect I could, specifically trying to kill ALL of them in the first round.
My point is...let the players make their own decisions. It's NOT your job as DM to "keep them alive" or "provide an even chance of success". It's your job to portray a believable world that your players can learn about and use that knowledge to make informed decisions.
PS: Stop "writing to the PC's capabilities" when designing adventures; that way only encourages self-entitled play where the Players expect to win anytime and every time they pick a fight. You do that and you will end up with whiny players who cry foul anytime one of their PC's suffers some significant drawback (like death, or loosing an arm or favourite magic item), claiming you "did it on purpose". And they would be right, because up until that point, you, as DM, *have* been building encounters you know they will win. You've more or less stated "you are the heroes and you will always win"...and then stabbed them in the back with "HA! Suckers! I put this in here to kill your PC's!". That's how they will interpret it anyway. Best to not ever let it get to that. Best to have your Players default thought process be "Oh, great! We're gonna die. Again...". Because when they DON'T die and they lived because they worked together, came up with solid plans, and played well....they will have smiles on their faces that will bring a tear to any Killer DM's eye! (Trust me...experience talking...
).
EDIT: PS: (this is after the XP from those below, but figured it's worth mentioning.
Last night we played. Two players couldn't make it (work related) so the three other players decided to not do our BECMI game and we'd just do something else. Decided on a quick 5e game. Anyway, as the PC's were exiting the Cairn Hills on their way to Greyhawk City, the camped for the night.I grab the Encounter Tables from the City of Greyhawk boxed set (or From the Ashes, can't remember), and roll. I get "Use Standard Encounter Table". So I grab the 5e book ...oohhhh...right. There are no Encounter By Terrain tables in 5e. Poop! I mention this out loud and go grab my Hackmaster books (d10,000 encounter charts by terrain and climate). One of my players, a 'temp' player who happens to be my young whippersnapper of a nephew (I think he's 18 or 19), who, up until playing with us this last month or so, has only played 5e, has a sort of 'confused' look.
Anyway, I then proceed to denigrate the "new style" of DM'ing preference of a DM custom-making or, usually, ignoring "Random Encounters" because they can somehow 'wreck' a game or some such nonsense. I point out that without a 'regular' (re: old skool) Random Encounter Table for some area, it removes the Players ability to engage in the world in a realistic and logical manner. If the PC's travel through some area multiple times and frequently encounter wolves, for example, then they can realistically expect that in a year when their PC's are all 12th level and come through the area they will encounter...wolves. Not ogres, giants, dragons or wandering demon lords. Wolves. Because this area of the world has a lot of wolves that hunt here. Always has, always will. The "encounter chart" doesn't change monsters just because the PC's gained levels. There was then a pause. ... ... And then he said "Huh. That makes sense. Cool". I got nods and 'yups' from the other two players and all us old farts kinda just smiled, knowing that this new player was beginning to see his view of RPG's and "encounters" in a whole new...er... old (?)... light. Kinda brings a tear to my eye.
^_^
Paul L. Ming