@Pass:
No, I think it's because a lot of other managas regard children way too highly. It's always the stubborn naive child who does some miraculous thing that aids the heroes in their quest. A 5 year old sneaks into the villain's lair and frees the heroes without getting caught. A 7 year old succesfully protects an injured hero from danger until he is back to perfect health. And it always happens to attract a young audience.
Oda depicts children as, just that, children. He doesn't follow the cliche of the eager unappreciated child who become a warrior that so many other series do, which is one of the reasons why I love Oda's story.
I'm not saying children are weak, but honestly, do you think any child could fend off an attack from any of the OP villains by themselves? (Goldenweek doesn't count)
I agree here with this statement. It always bothered me, not just with anime, but with children as a rule for young audiences they are made better then they end up. If we look at Ben10, we do see it happening, however Ben's human form is always depicted as a sharp contrast to his otherwise super alien forms.
And if you look at Oda's work, nothing really has changed with his dipiction of children since chapter 1. Even in the Alabasta arc, we see a small boy trying to join the rebellion only to be turned away because he was too weak. And woe and behold he gets beaten up (proven he is just too weak to fight villians). We also saw it with the Arlong arc with that boy who wanted to take on the Fishmen… And failed.
Thing is even when you do get a powerful kid like Robin they are thrown into situation they are too weak to take control of. Like Robin's story, remeber she moved a giant with her Hana Hana no mi powers yet was unable to do a thing about the situation at Ohana. It was beyond her control.
One of the reasons why I don't like the fillers Toei often makes is because they changed the Child-Adult formula around away from Oda's style...