My players want to hit multiple enemies with two handed weapons, like cleave, but I can't find anything in the rules. The most I've given them so far is on a critical roll; They can hit the enemies if they are close enough to each other i.e. shoulder to shoulder. This seems fair on smaller targets like kobolds. I figured maybe Unearthed Arcana guidance might be out there, but couldn't find any. I'm guessing this was to simplify rules or maybe they will expand it officially later... Any ideas?
The DMG provides an optional rule called "Cleaving through Creatures":
When a melee attack reduces an undamaged creature to 0 hit points, any excess damage from that attack might carry over to another creature nearby. (DMG 272)
Its use is recommended when the PC-s face many low-level enemies to speed up fights. Using it all the time with heavy or two-handed weapons will give those weapons a clear advantage. I would advise to make it a feat (also see below) or to impose some kind of penalty on the attack or damage rolls if you want to make it a basic property of big weapons.
Also, the Great Weapon Master feat (PHB 167) lets you make an additional attack as a bonus action if you finish an enemy. It is similar in mechanics to the above mentioned rule and could be "fluffed" as a cleave.
Eventually, martial-type characters (including some Bards, Warlocks, and Wizards)1 get the ability to attack twice in one Attack Action - Fighters get even more. There are also ways to get a single attack as a Bonus Action, and a few very specific ways to hit multiple targets (the Ranger has a few).
This is a deliberate design aspect, part of the game's action economy. Allowing a martial character to hit multiple enemies with a single attack as a general rule would disrupt this balance, making them more powerful than intended.
"Cleaving Through Creatures" (DMG p272) is an optional rule that effectively lets you spread excess damage from a killed creature to another adjacent creature. As an optional rule, using it will affect the aforementioned balance. Whether or not this is desirable is a subjective question each group must answer.
1Valor, Pact of the Blade, and Bladesingers respectively.