I believe sentience is a separate issue from intelligent; if they are not very intelligent than you will be able to simply outsmart them, trick them and deceive them into doing as you please.
How do we we successfully keep extremely lethal animals caged and controlled in a zoo, so safely that toddlers can be within striking distance of instant death and we are not concerned in the least? By outsmarting them; the snakes, spiders, insects and predators are just not smart enough to figure out how to get out of their traps without being killed. So they wander around their very tiny worlds looking for a way out, for the rest of their lives.
We are intelligent enough to design and analyze enclosures and foresee every possible means of escape they will attempt, and ensure those fail. But we still have access to the animals, can sedate them, treat their diseases, perform surgery upon them, and painlessly kill them when necessary.
Even if the animals are as smart as their creators, this can often be done, with somewhat less surety. We call such environments maximum security prisons.
But in that case, of course, the inmates know they are prisoners; whereas in zoo environments the animals may know they are stuck, but not realize their captivity was intentionally accomplished by humans; they may see their handlers and feeders as just part of their environment.
If your creations are highly intelligent with culture and language then religion and other mind-control protocols will eventually be worn down; it may take hundreds of lifetimes, but rigorous science, mathematics and general understanding of physics and "how the universe works" will accumulate and eventually dominate the society; thus atheism arises and cannot be stopped; it is inevitable given a scientific culture that demands either proofs or independent replication of experiments.
Here is how to secretly control a highly intelligent species.
I will put this, for the purpose of clarity and accessibility, as if we humans are the created species and the creators are aliens with highly advanced science. I presume you can translate this to your story premise without humans.
Suppose aliens are only as intellectually capable as the best of humans, but they have a billion year head start on science. Say they created us, circa 100,000 years ago, and have been watching us this whole time. Get up to about 1900 AD: It would be relatively easy for them to monitor us and know who is working on what and simply assassinate them, quietly. The general theory of relativity never sees the light: Why? Einstein went to school to learn physics; and they monitor every person that does that, for life. Einstein had to write his first paper, they saw that happening. And then Einstein the patent examiner is mugged and killed for the content of his thin wallet, his paper and works quietly vanish.
The aliens that created us can equally create avatars for themselves: completely biological and identical to a humans, but without any intellect: Their brain looks like a human brain, even under a microscope, but is laced with microscopic electronics so they can be 100% controlled by the aliens, and appear natural to real humans. The electronics are designed to dissolve if the avatar is killed or compromised, and kill the avatar in the process by what seems natural causes (heart attack, stroke, seizure). For near immortal aliens a human lifetime is a short blip; so they could even create such avatars as infants in the womb of a quietly abducted (or seduced) female, and let them be born and grow to adulthood with an interest in politics.
The alien technology and understanding let them turn any such avatar into a powerful and wealthy citizen of this planet.
So while murder is a tool they can employ as needed, they also have social tools: They can manipulate money and opportunities for research that guide the smartest of their creations into fields that do not threaten them.
So we have no Manhattan project unless they want one. We have no quantum physics without their approval. That research is just not funded, and the leading lights we recognize ended up working on things that did get funded: biology, mathematical puzzles, entertainment: Do not forget the aliens (in my premise) are much like humans emotionally, with very advanced tech, so they could still be very much entertained by their creations producing stories, humor and fiction, solving puzzles and in general being creative with machines and inventions. They love Shakespeare and Stephen King, they love Star Wars and Die Hard, Mozart, Rock and Roll and Hip Hop.
But they are stopped short, by avatar politicians, avatar businessmen, avatar assassins and other such interventionists, on any tech that could get the aliens discovered.
So there would be no Manhattan project, no quantum physics, no general relativity, no space exploration, unless they want it to happen. So we see this as our own stupidity getting in our way, without realizing the billionaires and dictators and corrupt politicians are all just avatars 100% controlled by the aliens to thwart any progress toward true understanding, and to force us into non-political solutions for our problems: e.g. they don't want us to find a political solution to global warming, for their own reasons they want to see us invent a technical solution. In this scenario, even Hitler and the Holocaust would have been their choice, perhaps to spur the complete rewrite of world politics that ensued after WW II.
You can control a smart species secretly; by secretly manipulating their economy, their research and their politics, through money, secret information obtained via hyper-advanced technology a million years ahead of our own, plus the use of avatars, and "explainable" assassination when necessary (i.e. it looks like an accident, disease, or plausible human-on-human violence).