Gfrktyl

I haven't played D&D for a while though recently I have started playing it with my kids. The group is made out of five. So far they have acted with wisdom and they have been lucky which allowed them to slay a Thundertree Green Dragon. Are there any rules for disassembling the dragon for the sake of doing things such as selling its meat and making armor from its scales?

There are no specific rules for disassembling a dragon or other creature to sell its meat or make armor from its scales.

Your mention of Thundertree leads me to guess that you're playing The Lost Mine of Phandelver, which implies a setting of The Forgotten Realms -- although not necessarily, you could forklift LMP into your own world and I am sure many do, and even if you want to remain as close to the default setting as possible, you end up having to make choices about how things work in your game.

I am not aware of any reference in 5E materials suggesting that the sale or consumption of dragon meat is a common, or even rare, thing.

There is official dragon scale mail armor, so it's reasonable such a thing could exist in your world. Making armor is a skilled activity, so maybe your characters have that skill, or maybe they don't. Maybe they need to find an armorer. Maybe any skilled armorer could make it, or maybe there's only one armorer in the entire world who knows how to make dragon scale armor. Xanathar's Guide to Everything (an official supplement) has optional rules that suggest crafting a very rare magic item would take 25 weeks and 20,000 gold, but it's up to you whether you consider that as reasonable in this case.

There are also numerous references to items made from dragon hide, dragon bone, or dragon scales which implies that there might be a good market for such items.

However, one does wonder how a trade in body parts of sentient creatures is perceived. Might a Dragonborn character take offense to a restaurant selling dragon steak? Might a dragon display the hide and bones of a human in its lair? "Yep, bagged this one back in '03, trying to sneak in to steal my treasure."

There are a lot of maybe's and might be's because this specific situation is not specifically accounted for in the rules. However, there is always the basic rule of D&D:

1. The DM describes the environment.


2. The players describe what they want to do.


3. The DM narrates the results of the adventurers’ actions.

It's perfectly reasonable for the DM to say, "Okay, you've got a dead dragon", and for the players to then say, "Can we butcher it and sell it?"

Then, you as the DM need to decide ...

...what you want to do at that point. You can deal with it as simply as "Yes, you're able to sell some meat and scales for a 1000 gold", or "No one wants dragon meat, ewww, but the armorer in the nearby town will make you a nifty set of dragonscale armor if he can keep the rest of the scales." Or whatever you want.

You can make dining on dragon meat to be the height of haute cuisine, or taboo, or anything in between. You can spend a great deal of time focusing on what to do with the dragon carcass, or you can dispense with it quickly and move on to something else.

Jack's answer is great and already explains that there are no official rules for that kind of thing made by WotC. Instead I want to show a way of handling such situations, as I do it with my players usually:

Players will want to take things apart, no matter their age-group. They have just killed a pack of wolves, or bested a dreadful owlbear and now they want their spoils. Some want to get cuts of meat for the victory meal, others want feathers & claws as decorations for their gear. The party wizard might want to fetch some rare magical ingredients for potions & spells. And the ranger or druid might wish to render the rest of the corpse so as not to waste the life they've taken.

In order to facilitate this, I have them roll Survival and/or Medicine checks then they make to take the carcass apart. I tend to use a scale with steps of 5, to measure their potential success and thus their possible spoils.

E.g. they want to take apart your dragon. They would make a Survival roll to make cuts of meat and take off the skin, and might wish to harvest magical ingredients with an additional Medicine roll:

Survival

0-5: You manage to cut off some great looking pieces of meat,
enough for more than one feast for you and your companions. Sadly in
the process you weren't exactly ..adept. Most of the dragon's skin
lies around in bits and pieces that might make for a good mug-warmer,
or saddle cushion - but you will have to find a master-skilled
armor-smith to make anything more from the remains.

5-15: With a few decisive cuts you cut the meat and skin off the
bones, it doesn't look pretty, but it's not that easy to practice
butchering a dragon anyway. You can secure enough edible meat and bits
to feed everyone at the banquet that will be held in your honour after
you return. A skilled armor-smith should be able to craft some
dragon-leather armor and comfy mittens for you easily.

15+: You dig into the carcass, an hour later you 'emerge' from your
task with a neatly cut up and cleaned out dragon. You left little to
no meat on the bones, and managed to keep the skin in one piece - head
attached. Imagine what a king might pay to have a dragon rug in their
throne-room!

Medicine

0-5: You can easily pull out some teeth and claws. But you somehow
managed to bash in the dragon's skull when trying to get hold of a
bigger tooth, the delicate eyes, tongue and brains of the dragon are
ruined for good...

5-15: You fill a whole bag with teeth and claws of the dragon. The
tongue and venom-gland you removed so adeptly will be highly sought
after by the local alchemist; it might easily fetch you a small chest
of gold or some curious potion(s) in return.

15+: The above results, and:

Going after a hunch you dig into the intestines of the beast. Inside
its stomach you find a bezoar the size of the barbarian's fist!

Our group had the same issue. There is no 5e resource for this that we could find, but we are using the 3.5 version of the Draconomicon as an aid for preparing a dragon for sale of its parts.