tl;dr: Divine Spark compares unfavorably at most levels versus unlimited use cantrips level 0 spells for a feature that uses a limited resource. While balance isn't the main thing WotC is concerned with for the playtest, if no tweaks are made this is likely to end up as an ability that feels bad to use.

I was curious how effective Divine Spark, when used to do damage, would be as a use of channel divinity, so I mathed out its damage numbers. For a baseline I compared it to the cantrips sacred flame and toll the dead (assuming d12s for damage), including the bonus from Blessed Strikes. Here's a table summarizing things, broken up by the levels where one of these scale. (To get average damage numbers I assumed a 65% chance for the foe to fail the save, regardless of save type.)

To make the comparison clearer, here's the percentage increase of Divine Spark versus the two cantrips at various levels. Negative values mean toll the dead performs better at those levels.

In brief: Divine Spark is much better at levels 1 through 4 before the cantrips get any scaling. After that it drops off sharply, especially in comparison to toll the dead. At times toll the dead even out-damages Divine Spark! Confining the comparison to sacred flame makes Divine Spark look better. But nonetheless you are spending a limited resource—which could instead be used for healing or turning undead or whatever your subclass gives you—to do single target damage that isn't much better than a mid-tier attack cantrip.

Of course there's limitations to any sort of white room theorizing, so let's discuss them.

First off, the tables don't show that Divine Spark has better reliability. Since it does half damage on a successful save, it's never going to give you an action where you do zero damage. This also means that Divine Spark compares even more favorably against foes with higher saves. If the chance of a failed save is lower than the 65% I assumed for the table, Divine Spark's damage numbers go down at a lower rate than the cantrips' damage numbers. On the other hand, if the chance of a failed save is lower than 65% then the cantrips scale better.

On the topic of saves, the three abilities target different saves. Unfortunately for Divine Spark, it targets Constitution. Monsters tend to have decent to high Constitution scores, whereas low saves in Dexterity (for sacred flame) or Wisdom (for toll the dead) are more common. So in practice, sacred flame and toll the dead perform better than the tables show.

Another consideration is range. Divine Spark only has a range of 30 feet. Sacred flame and toll the dead have a range of 60 feet. How much this matter depends on your game. But if your DM is like me and has more encounters outside cramped dungeons than inside, then you'll regularly be in situations where the longer range matters.

There's also the damage type. Necrotic resistance/immunity is common among some monster types, so toll the dead won't be a good choice there. However, sacred flame has the same damage type as Divine Spark, and it's easy for a cleric to keep both prepared. So this is only a small point in Divine Spark's favor.

Finally, there's the point that I'm comparing the playtest material to 5e material. There's always the possibility that toll the dead gets a nerf or never gets ported over to D&DOne. Or maybe cantrips as a whole will be tweaked in one direction or the other. But assuming cantrips in D&DOne work very similar to how they do in 5e, Divine Spark is undertuned.