It's ridiculous for him to be one of the strongest Shinigami when he was like under 100 years (seems to be that way).
Hitsugaya's at least 50 years old, so while Kenpachi looked like a child, he had probably been around for decades at the very least.
Think about this. Hitsugaya comes from the safest district of the Rukongai and Kenpachi comes from the absolute worst district of the Rukongai where you either have to fight to survive or find someone who'll fight for your survival.
Despite having a very sheltered life (idyllic by Rukongai standards) and experiencing nothing worse than people ignoring him or being rude to him, Hitsugaya was still captain-class and on the verge of manifesting his zanpakutou spirit without any formal intervention. And that was without any survival pressure on him. At the other end of the scale, we have Kenpachi living in an environment where he's got to use his power to survive from an early age and build himself to a point where nothing in that hellhole can ever threaten him because he's become so very strong. That's before we get into the vast difference in their natures (Hitsugaya is more peace-loving, Kenpachi loves looking for fights) and also before we get into the question of the origin of the zanpakutou which is seems to be rather relevant to Zaraki's past (since he had it before he met Unohana).
If Hitsugaya is guaranteed to be stronger than the current captain-commander in just a century's time, having come from such a pressure-free background, then a much wilder personality from a much more vicious background could easily have been doing better than that at the same age.
Hitsugaya has already set the ballpark for something like this to happen. So has Ichigo's insight into Child!Aizen. Even Gin with his demolishing of a third seat after only a year training in the academy and Yachiru being such a very young vice-captain.
This whole issue about a child Kenpachi already being stronger than 'the Divisions' Kenpachi is relative to nothing but Unohana and Zaraki (for instance, it tells us absolutely nothing about either Unohana or Zaraki relative to someone like Aizen), and the chapter went out of its way to point out what an apathetic state Unohana had fallen into by the time she met Zaraki. We also know Zaraki hit his limit when fighting Unohana: that, in fact, is the source of the problem between them. It's not like Unohana is saying Zaraki easily surpassed her back then. She's saying that by him hitting his very limit (his limit at that particular time) back then he could have killed her if he had wanted to but that it's *now* (in this present time) that he needs to (and will) surpass her.
Kenpachi having fought Unohana as a child? No, I'm not thrilled by this. I'm not pre-disposed to like child characters, so learning this isn't something that I'm particularly enjoying. That said, it's not as though the story hasn't already set up the possibility of something like this happening. It's also not as if the story has currently told us anything relative to a power level we do know a bit about (Aizen's level of power): how did Unohana and Kenpachi rate back then compared to Aizen pre-timeskip? Kubo's avoided telling us that.
One other thing though: I've seen people saying this is pandering to the novel. My reaction on reading the chapter is that it actually debunks what we know of the novel. The novel said Kenpachi adjusting his reiatsu to his enemy's level is his power but Kubo is telling us it's his prison (implying his power is something else, even if it's related to this, it's not exactly this). It's like saying Narita picked up on something Kubo had been doing but instead of putting 2+2 together to get 4, Narita actually got 5. From what I've heard about the novel, this chapter doesn't seem to legitimise it at all, but it does suggest Narita ran off in the wrong direction with a legitimate concept.
Yes, Kubo is setting up Zaraki to be the strongest captain of the current generation (and in the process surpass the Kenpachi of the strongest generation), however Kubo harping on constantly about the first generation being the strongest generation is hinting at the strong possibility that Kubo's aiming for the current generation of captains to surpass that generation of captains.
After all, the story is so far basically saying that the strongest generation of Division captains couldn't really do anything about the Vandenreich (on a permanent basis) suggesting that for the Vandenreich issue to be truly resolved, the current generation of captains has to achieve what the original generation failed to do. That means this generation has to surpass the original generation in some fashion or other. It just happens that the Kenpachi is a position relative to other captains and that the original Kenpachi is still alive so we get a direct, unsubtle comparison.