Your party is not comprised of murderous specicist cretins that kill members of a different culture creating species based on the species alone but instead forge friendships. If that bothers you, look at My players went from 100% murderous cretins to 100% nonviolent diplomats; how can I achieve a middle ground?
Win = XP
This behavior alone is worth XP, and the one and the best thing I see in 5E is that this XP shall be exactly the same amount as if they had slain them. And it's even better for the players, as they didn't have to spend too many resources (maybe rations or what they gifted to sway them), but the healing potions and spells are still there.
Roll/Role with it!
Define your Goblins!
A little caveat starting here: The standard description of alignment is something that I often ignore if it doesn't match my liking. A race that is evil isn't necessary a bunch of murderous cretins, it has culture and customs that paint it evil, yet still, they manage to forge a culture. Define what makes your Goblins evil, and work out how they behave based on that. To illustrate, I provide a rough picture of my gobos and orcs, both of them having their evil culture:
For my goblins their culture is based on raiding and slavery of other races, trickery and the right of the strongTM, they only honor their word so much that they fear their opponent. But they value the children of their whole tribe, as they are the future warriors, and they value their elders, for staying alive for so long means they must be some of the most badass among them. Atop that, they are pretty much believing in no god and instead spiritistic with an anchestor cults, believing that to apease the spirits of the deceased one has to sacrifice them the blood of animals or strangers, leading to occasional raids just to get hands on someone ot apease the dead.
On the counterside, my Orcs are deeply entrenched in a blood cult that puts shamans into power as they conduct the rites to their blood god and favors warriors. They are also in some kind of hierarchic class system: A double-tip of a high-priest and a warlord can unite several tribes, each tribe has his priest(s) and chieftain and below are warriors that again are above the craftsmen, who still are above the (labor) slaves to mine and farm and then comes the lifestock, which includes females. Service to their god is done by raiding, duels of honor (for rank, slaves and lifestock) and blood sacrifice of as many captives as gruesome as possible.
This further reading is to TV-Tropes Warning material, but still worthwile:
The WorldTM behaves accordingly!
Now, a little pet peeve of me is to try to keep games realistic in some way. They did spare these goblins.
The word about them might spread among the goblins and when they meet another group, they might offer them help for just a little help with these pesky murderous cretins that had assaulted one of their hunting parties last week and now threaten their burrow - and they may keep whatever they slay even!1
They might be approached by a man in ragged robes, that asks them to help to relovate this cart full of children to a certain orphan house, because their parents were slain by a bunch of murderous specicist cretins, and these heroes clearly would do such a good deed to protect the innocent... goblin children to be relocated to an orphan house run by a dragon2.
But of course the general populus might not like the happenings, if they learn of this: townfolk are simpleminded. They know of simple things and think simple things: They are good, the others are evil. Raiders are evil. Goblins are evil. Who helps the evil ones must himself be evil. When evil comes, they have to defend their homes. To defend their homes, they send a message to the evil ones. To send a message of this kind, one hangs an evil one at the gallows and lets it rot. How handy that pitchforks and torches are never far in a farming community built as a bastion of the Good Law!3
1 - of course, this bunch of cretins is a typical group of adventurers that call themselves heroes. A cleric, a rogue, a fighter and a wizard for example. Pretty much a mirror duel, if they can't appease them too.
2 - Some examples: Matafleur from the Dragonlance series: old, half-blind, a little deranged but loves children so much that she doesn't care for their species. Don't dare to try to harm them or she might unleash the power of her ancient being! Or she might be a more sinister Mrs. Grindtooth from Jim Button and Luke the Engine Driver, chaining her kids to the school tables and forcing them to learn algebra and foreign languages
3 - Lawful Good doesn't mean all are nice and inclusive. That's True Neutral! Now, cue an Angry Townfolk encounter for extra XP! Let's see how they deal with members of their species that have the lawful simple, eh, lawful good alignment that try to burn them for small acts percieved as unlafwul/evil by them.