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?
[RPG] Would giving the halfling player characters darkvision significantly unbalance the game
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.