I helped raise a pair of infant kittens last year (my wife found them abandoned at 3-4 days of age). We gave them kitten milk replacement from a vet. Because the milk replacement wasn't very high quality I tried giving them meat at a very young age (I can't remember exactly how young, but they they were too young to chew anyway, probably ~3 weeks old).
Specifically, I started them on very small slivers of raw chicken heart, they would suck the sliver of meat into their mouth and suck on it - I think they wanted to suck it in, but it would get caught on their tiny teeth. After a couple of days they learned how to eat it properly and pretty quickly lost interest in the milk replacement, at that point I also started feeding them a wet food for kittens to ensure they got the nutrients they needed. They gained weight much faster once they were eating food. Overall they were "weaned" from milk at a significantly younger age than kittens with a mother would be, but they still thrived and are both now very large and healthy cats.
Tangentially related we also found a severely emaciated young kitten at about the same time (probably 4-6 weeks old), which I also initially fed slivers of raw chicken heart. It seems to be particularly tempting for them.
If/once they are big enough to chew you can try giving them raw chicken necks or wings (in pieces too large to swallow whole), even small kittens are remarkably effective at chewing the meat from bones and it will help them with teething and jaw development. They also make adorable growling noises while chewing.
The final thing to note is that the first few months of a kittens life are extremely formative in terms of what it grows up believing food is. While cats can be fussy, a young kitten will try eating almost anything and it will tend to remember what it ate as a kitten. Since I didn't intend to keep any of these kittens, I fed them all a broad range of food: raw meat, raw bones, wet food and kibble, so as adults they would eat anything their forever owner deemed fit to feed them.
This conditioning also makes it easier to give cats medicine or special medical diets, a concrete example of this is that these kittens were extremely hard for the vet to handle (being hand-raised by humans made them uppity) and he couldn't give them squirt-in-the-mouth oral medicine due to all the squirming and wriggling. I asked for pills instead and just stuffed a pill into a chicken heart, which the kitten would swallow whole, oblivious to just being fed medicine. We have another cat who obviously only ever ate kibble as a kitten and she is a nightmare to give pills to as she never mastered the art of snaking down whole pieces of meat and is suspicious of things that smell funny. So this is your chance to condition the kittens into being easy to feed and medicine cats.