There's actually two parts to your question:
Do Akkis (fiends/ogres) have abnormal strength?
There's a bit of a disconnect here. Maria and Mamoru's child is not actually an Akki as mentioned in the last episode. The akki "K" in the flashback is actually a grown up teenager. So he could very well be that strong in the first place.
So the real question is: "Why is Maria and Mamoru's child so strong?"
This assertion does seem to be true since the kid is able to do precision kills while normal children of the same age have trouble levitating a ball.
A speculative answer would be to go back to the fact that Cantus is a power of the imagination. It's possible that Maria and Mamoru's child, having grown up outside the village and influence of humans, has a completely different mindset and thus different imaginative potential.
Why was Maria and Mamoru's child able to defeat Shisei Kaburagi (the strongest Cantus user) at such a young age?
This is simply because Shisei, a normal human Cantus user, is restricted by attack inhibition.
So regardless of how powerful he is, he cannot hurt the child. On the other hand, the child can freely attack Shisei. Note Shisei does attempt to defend himself since he manages to protect himself against the rocks that the child threw at him. But once the child used his/her Cantus directly on Shisei, Shisei was as good as dead.
In other words, if you tied a 200 pound wrestler against a wall and let a 10-year-old kick him in the balls repeatedly, eventually the kid is gonna win.