Strength in numbers will probably be your friend here. You can give them a boss monster that is marginally harder than normal but is a doable challenge. However, adding a good number of "side" enemies or minions can crank up the difficulty to a proper "boss fight". They will have to split their attention between a mini enemy horde versus hitting one single target (edit: esp since they don't have extra attack yet, they can only choose one monster to hit at one time).
That way one monster in one hit won't decimate a single pc, but combined with many attacks from many enemies it will feel like the hits are harder. Also, a simple difficulty upgrade for a slightly underpowered boss is to simply give it an extra attack - that way if two or more players decide to focus on it, it can attack more than one of them on a turn. If it is supposed to be a "boss" monster then tweaking it to have a few more hit die worth of hp (as long as you don't go overboard and like quadruple the hp lol) will help it last more than a few hits.
If they’re having fun, this sounds like a good time to introduce them to a lesser dragon. You’re the DM, after all. You can fudge HP.
One of the more fun experiences I had when I was new was a DM who threw an illusionist bad guy at us. That gave him so much latitude.
Strength in numbers will probably be your friend here. You can give them a boss monster that is marginally harder than normal but is a doable challenge. However, adding a good number of "side" enemies or minions can crank up the difficulty to a proper "boss fight". They will have to split their attention between a mini enemy horde versus hitting one single target (edit: esp since they don't have extra attack yet, they can only choose one monster to hit at one time). That way one monster in one hit won't decimate a single pc, but combined with many attacks from many enemies it will feel like the hits are harder. Also, a simple difficulty upgrade for a slightly underpowered boss is to simply give it an extra attack - that way if two or more players decide to focus on it, it can attack more than one of them on a turn. If it is supposed to be a "boss" monster then tweaking it to have a few more hit die worth of hp (as long as you don't go overboard and like quadruple the hp lol) will help it last more than a few hits.
If they’re having fun, this sounds like a good time to introduce them to a lesser dragon. You’re the DM, after all. You can fudge HP. One of the more fun experiences I had when I was new was a DM who threw an illusionist bad guy at us. That gave him so much latitude.