@Trexxe It's explained better in the novel this manga is based off of; his power is to corrupt the holy power within them by merging his essence as the Demon Lord of [Evil] with other creatures and in doing so demonifies the children so spawned. You can't demonify a demon, so doing it with his kids only increases his numbers (which he did, enthusiastically), not their strengths. Basically, the power boost comes from the corruption process transforming the base model into a demonic creature variant, not from just spawning children in general. It is specifically outlined in Chapter 9, Volume 3, if you want to read it.