I admit my main characters as children were indeed still overpowered at the time because one of them (10 years before the main story so about 9) jump kicked the face of a giant behemoth and killed him in a single blow despite him being heavily feared among the other warriors. :lol 
The story clearly has some anime influences and characters in the multiverse can grow to insane levels of power like dragon ball z although usually on the level of taking on armies by themselves rather than blowing up planets. That being said, if people can be that powerful then it only makes sense they'd be overpowered even as children when put up against normal folk. It's harder to explain in settings where everyone has a more consistent power level, in which case the child wouldn't have much of a chance, if any, unless a prodigy at around 8 or so like I prefer for youngest.