I have four PCs who have just reached level 4. They are an elf and a half-elf, who have darkvision, and two halflings, who do not. A significant portion of their adventuring occurs at night or in dark places. I as a DM am getting tired of adjudicating light and vision all the time. I'd like to come up with a pretense to give both the halflings goggles of night as a quest reward, so we wouldn't have to worry about having characters without darkvision anymore. Would this break game balance?
Would giving your players uncommon items break the game? No
The rules for this item exist, so you're not adding anything new to the game. Giving uncommon magic items at level 4 is not out of whack either. Mechanically, you're fine; this isn't going to break anything.
Compensate your elves
On the other hand, your elven players chose a race that gave them Darkvision. If you just hand out Darkvision to everyone, they will likely (rightly) feel a little left out. However, this is easy enough to remedy - give them each some magic item at around the same time as you hand out the Goggles of Night and you'll be fair while also solving your problem.
Short answer; no, it wouldn't break anything
It just means you're ignoring lighting conditions so the game is more fun for everyone involved.
Should halflings need to give up something to compensate? Or does everyone now also, say, have Stout Resilience to poisons? What if a human NPC joined the party? Do they get darkvision too?
Some mechanics are easily overlooked, but there needs to be trade-offs in races so you get some good, some bad. As the saying goes, "If everyone is super, then no one is super."
Are the elven players back-seat driving the adventure? It's pretty common for players to want to only do things when their perks are most advantageous. EG: the elves have darkvision, so they'd want to push to adventure at night and in dark dungeons w/o torches or light in order to maximize their vision and minimize detection (when playing thieves).
But, if the elven players know that maximizes their perks, while putting the halflings at a disadvantage.. that's selfish. A strong player personality may be strong-arming the adventure to the benefit of their own character, but the detriment of others.
EG: played in one campaign where the majority fighters in the group wanted to keep pushing on and adventuring even though the wizard and cleric were depleted on spells. There was a strong player personality on the fighter side of things, and they just didn't care about the spell-casters needing to stop and replenish spells. That player would also cause fights knowing the spell casters couldn't take on the battle. We gave that guy a talking to, but that just made him more of a jerk. So, we eventually booted him from game play. He protested, so GM said if he wanted to come back he could only play a spell caster, so he could see how it felt constantly being pushed into uncomfortable situations by other players. He didn't like that. Left our group. Guy was a jerk.
So, my idea is .. if you're trying to help out the halflings because a strong player personality in the elves keeps pushing for situations that put them at an disadvantage, that is a player issue, not a game mechanics issue. If you try to fix a player issue with game mechanics, then it just rewards the player being a jerk, and they'll do more selfish stuff and expect you to dole out more game mechanics to make up for it in never-ending cycle.
Just something to think about.