The older cat makes it impossible for you to blindly rely on the cats to fairly distribute the food among themselves, which means you need to be more involved in this process. There is a clash between your current feeding strategy and the ability of your cat(s) to self-regulate their food consumption.
If the older cat is eating more than the food you've "assigned" them, this either means you are not providing enough food for the older cat, or that the older cat is overeating.
If the former (I don't think this is the case, but I'm pointing it out as it is possible), the straightforward solution is to provide the appropriate amount of food. If you cannot provide the appropriate amount of food for the amount of cats you have, you have to reconsider how many cats you keep as pets.
You are currently feeding the cats on a freely available food basis, i.e. the bowl of food is available for long term and you replenish the food once in a while. In a multi-cat household, this relies on all cats being capable of self-regulating the amount of food they eat, as there will be more food available than one specific cat should eat.
If your cat overeats, it therefore has no reasonable ability to self-regulate, so you simply cannot keep food for multiple cats freely available without oversight and without the older cat being able to overeat.
The simplest solution here is to have specific meal times, and to not make the food bowls available outside of those times.
The older cat can eat from two bowls, but it cannot eat from two bowls at the same time. Therefore, by keeping the bowl time limited, each cat will be eating at the same time, each to a bowl (or sharing a bowl, if they are so inclined).
Because bowl availability is limited, this also makes it easier for you to intervene when needed. If the older cat eats significantly faster and outpaces the kitten, you might need to keep the older cat away from the kitten when it is still eating.
Depending on the older cat's behavior, this can be achieved using simple verbal feedback (it works for our cats), or by physically blocking the older cat (with your foot, or by putting it in another room).
Anecdotally, we were faced with this issue. Our first two cats were sisters who grew up together, so they always shared food. But the third cat, a street rescue, had gone hungry for much of its life and would easily eat all three portions if left unchecked.
By feeding them at the same time, he wasn't able to butt in to several bowls at once, and the girls could eat unhindered while he was busy on his own bowl.
We sometimes had to stop him from finishing early and butting into the other bowls, but we were already needing to train his other behaviors, so this was no different. Initially, we physically blocked him, but over time he learned to listen to verbal cues, being told "NO" is usually enough to get him to back off.
In his defense, he is much more active than the girls are and has always been skinnier than them (even years after he started living with us), even though he tends to eat significantly more. He's not actually fattening, he genuinely needs the food for energy.
Because he is a natural big eater (not leading to obesity) we do allow him to finish the girls' leftovers after they voluntarily walked away from their bowl, because that is considered "giving up" the remaining food. If your cat is eating to the point of obesity, you might not want to allow them finishing the leftovers.
Over time (a few months), we no longer needed to intervene here. He would sit and stare at the other bowls, well aware that he needed to wait for them to leave.