And I keep thinking that the demon lord making his children fight a Battle Royale is already an insane concept. They're going to lose lots of fighting potential in the war against human just to ensure that he has a single heir.
It's even more stupid when the fight start as some children are so young (the MC is 8 here, physically at least) and barely know their own powers, compared to the older ones who probably have at least a decade of experience more. He did ensure that she does have at least some degree of fighting power, but chances are clearly unequal. How much potential the children have is pointless, so he might end up with the weakest one in charge just because he has a several-years headstart.
The only reason the MC stands a chance is that reincarnation which gives "her" the maturity of an adult. Some of it. Still no fighting experience though.
This reminds me of a series of YouTube videos "Villains Too Stupid To Win". This demon lord definitely qualifies.