When should you start talking with kids (...)?
Parents should start talking with their offspring before birth, the fetus responds to sounds starting around 23 weeks into pregnancy.
how much could you influence them like this?
The baby will be able to recognize the voices, sounds and melodies. Note that the baby would not recognize or have recollection of any spoken words... not only because it is not creating permanent memories yet, but because of the acoustics of the womb.
What? Do you mean to talk to them using a high pitch voice, inaudible to adults, to indoctrinate them into your crazy regime?
Well, in that case: Never.
Speech is not only harder to understand on the higher pitch, but you do not really want to target the speech centers of the brain at all.
Here is what you do:
Start by using the tone as reward system. Placing it in multiple applications and in particular in video games that the kids play, and set to sound when thing work correctly. For example, beep when the kid enters a new area, when solves a puzzle, etc. Bonus points if the parents also praise them for these things.
The idea here is to have the kid associate that doing that behavior is a good thing. And that they hear that tone when they do a good thing.
Eventually the kids will have a dopamine boost when they hear these tones. This is what you want.
Even when the ability of the children to hear those tones goes away, the positive reinforcement they had with the places and things where they did hear them does not.
You can do a bit more by using melodies. You reward specific actions that follow the behavior you want using melodies on those high frequency tones.
You can use those melodies in the real world to direct the kids. Hide them in other media (like actual music, for example) or in news reports, or have them play in the background in certain locations.
Humans can recognize melodies even if played in a different pitch (bearing the occasional amusia). This means that you can use even audible versions of these melodies. Although this increases the risk of starting a conversation about them.
You want to start this process when the kid is three years old, at this point the child can start handling a tablet or similar device, is willing to explore and experiment, doesn’t really question the world, and a lot of people have their oldest memory around this age.