RAW for Sanctuary breaking are as follows:
If the warded creature makes an attack or casts a spell that affects an enemy creature, this spell ends.
Is channel divinity "casting a spell"?
Channel Divinity to turn undead is not casting a spell or making an attack:
At 2nd level, you gain the ability to channel divine energy directly from your deity, using that energy to fuel magical effects.
As an action, you present your holy symbol and speak a prayer censuring the undead. Each undead that can see or hear you within 30 feet of you must make a Wisdom saving throw...
If it were casting a spell, the rules would say something like "Using this feature allows you to cast blah blah blah".
From the Player's Handbook, p. 202:
When a character casts any spell, the same basic rules are followed, regardless of the character’s class or the spell’s effects.
Each spell description begins with a block of information, including the spell’s name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell’s effect.
The class feature description makes no mention of casting a spell. Further, none of this information is present in the description of Channel Divinity.
And while it isn't an official ruling, Jeremy Crawford, the lead rules designer for D&D, was asked:
Does Dispel Magic auto work vs Channel Divinity powers?
And he opined on twitter that:
A Channel Divinity option like the paladin's Sacred Weapon isn't a spell.
If it were an attack, the description would say "make an attack roll" or "make a spell attack roll".
From the Player's Handbook, p. 194:
If there’s ever any question whether something you’re doing counts as an attack, the rule is simple: if you’re making an attack roll, you’re making an attack.
That's up to your DM. But maybe this part of the Turn Undead feature's description can help:
As an action, you present your holy symbol and speak a prayer censuring the undead.
Your DM might view this as an action that is primarily defensive in nature. In that light it might make sense that it doesn't break the sanctuary.
Or, perhaps it is simply a plea for help from your deity. Maybe it is not you, but your deity that acts. You are merely a vessel to channel that power. Maybe, because you took no direct offensive action, the sanctuary holds.
Or, your group might reasonably agree that this doesn't make sense, and house rule that it breaks sanctuary.
But, ultimately, it's up to your DM to narrate the action.
As Doval pointed out in the comments, Jeremy Crawford has also expressed on Twitter that despite the way the Sanctuary spell was worded, the intent was that other actions should also break the sanctuary. Subsequently, the publisher changed the wording for the Sanctuary spell in the November 2018 Errata.
Before the errata was publish, Jeremy Crawford gave the following responses to questions about the design of Sanctuary:
Q: Cast witch bolt and in the next turn cast sanctuary. The automatic damage from witch bolt cancel sanctuary?
A: RAW: witch bolt exploits a loophole in sanctuary. RAI: witch bolt's damage ends sanctuary.
Q: You first cast spirit guardians and then sanctuary. Does sanctuary end when spirit guardians deals damage to enemies?
A: RAW: Sanctuary isn't stopped if an already-cast spell deals damage. RAI: A damaging activity ends the spell.
If the warded creature makes an attack, casts a spell that affects an enemy, or deals damage to another creature, this spell ends.
While this would not change the interaction between Sanctuary and Turn Dead, it would mean that Witch Bolt and Spirit Guardians can now break Sanctuary.
This is a bit of a loophole, so a DM may judge otherwise, but strictly speaking Channel Divinity is not Casting a Spell, nor does it involve an attack roll.
From PHB, pp58-59
At 2nd level, you gain the ability to channel divine energy directly from your deity, using that energy to fuel magical effects... As an action, you present your holy symbol and speak a prayer censuring the undead. Each undead that can see or hear you within 30 feet of you must make a Wisdom saving throw...
It is solely a Class Ability that uses an Action and therefore sanctuary remains active.
In addition, Sanctuary is a Cleric Domain spell (and only Cleric Domain), so if they had wanted Channel Divinity to end it, that would have been easy thing to add.
NOTE: War Domain Channel Divinity - Guided Strike is an exemption. This involves an attack roll and would end Sanctuary.
RAW, it looks like a loophole. However, note that it does not specifically call out the "Attack" or "Cast a Spell" actions, but a more general "If the warded creature makes an attack or casts a spell that affects an enemy creature ...". Turning Undead is not the "Attack" action, but it is an attack that affects an enemy creature. Also, Turning Undead (and Channel Divinity actions in general) are not specifically spells, but they are specifically magical effects.
RAI, the whole idea of the breaking Sanctuary (or Invisibility, or hiding, etc.) is to not allow characters to take offensive actions and remain unseen. This has been consistent in all editions of D&D/AD&D.
Rationally, turning undead requires you present yourself and project your power to affect them -- "As an action, you present your holy symbol and speak a prayer censuring the undead. Each undead that can see or hear you within 30 feet of you must make a Wisdom saving throw ..." is consistent with this idea. The "see or hear you" part does admittedly leave room for interpretation, but you can't really "present your holy symbol" if the enemy is unable to perceive you.
I would rule that turning undead is effectively an attack for this purpose, and breaks Sanctuary/Invisibility/hiding/etc.
Full disclosure, I'm the DM that made the ruling Matt Daumen mentioned in the comments. At this point I now see it going either way, but wanted to explain the rationale for the ruling
One way of interpreting clerical spellcasting is that it is itself a channeling of divine power, using rituals prescribed by the deity and the church. Channel Divinity, on the other hand, is a special boon granted to those of exceptionally strong faith. It allows those priests who have proven their faith to more directly ask for intervention in an unstructured, improvised manner. Since most priests of a given deity may never achieve the strength of faith necessary even to cast spells, likely the first time a cleric uses their Channel Divinity power, it is an act of desperation. Despite their faith they may be completely surprised that there god would intervene directly without the structure of a spell, but simply through the strength of that faith.