My name is Grimdarkus Prime, and I am a madman. I have dreams of world conquest and plan to build an army made up of demonic soldiers that are stronger and faster than regular humans. To do this, I have invented what are known as behelits, vast artificial wombs made up of the corpses of demons stitched together. These behelits are infused with demonic energy from the warp, a reflection of humanity's darkest fears and desires. This is a realm where emotions and nightmares are sentient, and its energies corrupt the human form. I will use this to create the perfect blend of man and demon, and make a vast army subject to my will.
I plan to use the prisoners and populations taken during conquest to make this army. Tens of thousands of people have been enslaved and given the honor of being candidates for this process. Adult humans have fully grown into their bodies and are no longer changing on a genetic level. Most devolve into chaos spawn, which are monstrosities with horrible mutations and are uncontrollable. This makes them little better than canon fodder, and they die in a number of weeks if nothing else kills them. Children seem like a better candidate, since they are blank slates and are still growing. However, they are too weak to survive the infusion of demon energy, and either drown during the process or are physically destroyed by the process.
How can I solve this conundrum and increase the success rate of my process in order to build this army?