I've suggested this for a few timelines (including
@Joshuapooleanox 's Cesare Borgia TL), it would be a powerful tool for ensuring cultural shift in conquered territories.
Fundamentally you're interested in isolating them from their old culture and more-or-less brainwashing them to make them into soldiers and settlers for securing territories, or bureaucrats for administration. Being slaves is pretty optional. You're brainwashing children into being a loyal corps.
You then have two issues. Risks of lingering cultural sympathies, at which point you have to consider the value of deploying them in their cultural homeland, or settling them there as their retirement. The risk of defection is reduced if you take them young enough as they become akin to a modern Third Culture Kid, and look after them well. That might help when it comes to resettling conquered territories - as you aren't defining them by their source culture, reducing their chances of identifying as such and turning against you.
I think
@Yanez de Gomera is right that the ERE is a prime candidate, but this entire process could be a way to build a centralised state as well - an example where this could be done is the Norman Kingdom of Africa. If they immediately started taking strong children, en mass, for this process - in the interceding 15-20 years, they can raise these bureaucrats/soldiers/settlers in Catholic Monasteries, funded for this purpose. At which point, they can use them to replace the Muslim princes they had in place, and replace them with a die-hard loyal bureaucracy and professional army that is essentially a seperate class to the Nobles and Peasants (in the same way that many see Public Sector workers as completely different to Private Sector workers).
A PoD - this is exactly what happens with the invasion of Africa, with the Sicilians setting up new monasteries in Sicily and Italy, where those monks are instructed to raise the children given to them as strong, literate Christians, loyal to God and King. Those monasteries may at first be small simple villages that are building the monastery at first, but that works to the goal - the church is literally being built by these children. At the age of 14 you take the boys and turn them into soldiers. If you choose to take girls as well, you raise them as bureaucrats, but also as book-copiers. By age 18-20 the Sicilians have a substantial population, and 'queue' of troops and bureaucrats to build a centralised bureaucracy in the conquered territory, leaving the feudal system intact at home. When the time is right, or when nobles rebel, or break the law, you can use these same forces to occupy and keep their lands.
When the soldiers are too old, you retire them from military service, with a plot of land, which they can choose to sell back to the King, or farm. Suddenly all those childhood skills are immediately useful again - alongside their education and strength.
Don't get me wrong, this is
terrifying, and isn't far removed from ethnic cleansing - but I see no reason that any feudal state couldn't achieve this route to centralisation. The hard part is maintaining control during those first 10-15 years where you're kidnapping children at sword-point, be it with silver tongues and promises of educating children, or making an example of enough parents that people stop resisting the process. After that you want to make it something families will offer their children to, because they can't afford to raise them, or because they think it is good. That way you can get recruits from long-conquered territories and 'homeland' territories.
Side note : this could also be done as a way to reverse barbary raids. You can target coastal cities and raid them for infants.