So many questions brought up in the comments. For fun, let's see if we can hash out some possible answers:
1) "If security is that lax, then how is it the demons haven't been invaded and lost the war already?" Okay, first of all, let's reiterate something that was stated earlier in this series: the Demon Kingdom is a pit. No, seriously. Akari referred to the demon kingdom as "living on hard mode" in the first couple chapters. They didn't even have arable land until somehow Deux rejuvenated it. So, no one would likely want to invade and take what they have. Regarding the conflict with the humans, there are two possibilities: 1) The conflict is largely ideological with both sides seeing the other as "evil" and thus have no interest in the others' land and resources, or more likely 2) When the King took the Demon Lord's mother, he already got exactly what he wanted and the humans have been fighting a defensive war against the demons who are trying to take her back.
2) "Why did they abduct Akari when it would upset these incredibly powerful children?" They probably don't know what the children are capable of yet. Besides, it might just be that they wanted Akari. (This is going to sound a bit skeevy but we're talking about a manga where the main character is a teenage girl whose base ability is the ability to pop out babies, so the subtext is already a bit iffy anyway) The Elf King and the Dwarf King may want to use Akari's ability to produce strong heirs themselves. If they can seduce her into birthing their own children, they could possibly tip the balance in their own favor.
3) "Are the children's abilities for war or building kingdoms?" Potentially both. From the beginning, they've made the point of showing that the Demon Kingdom is "undeveloped" ("Living on hard mode") and even contrasting it with the humans' more developed town. The kids could do a lot to improve things. But they also had the demons make a point of saying that the children are meant for war. How those skills are used are up to Akari's influence on them. It kind of follows real life too. The same modern chemistry that created medicine also created mustard gas.
Anyway, fun chapter. I hope the little cuties save their Mama. I kind of hope they find a way to do it peacefully too (no reason to make a war worse). And finally, the builder child has been born! Someone can finally invent the demons' some indoor plumbing!