This just makes me think of what Dadan said to Ace in this very chapter about how weak he is and how he would be 'killed and forgotten the next day' if he went off to get revenge. Ace and Sabo were strong kids, but they were still kids. And even though this is One Piece and there are lots of fantastical things that happen, we've seen that being strong isn't always enough. Remember, Ace was strong as Hell as an adult and still got taken out like a chump by both Blackbeard and Sakazuki.
You're not seeing the big picture.
Yes they're children. Ace and a frigging she-male silverback gorilla supposedly barely took out some shit-hole pirate that dicked around in East Blue.
Of course he was comparatively 'weak' as a kid.
But he was still hard as nails for his age. Look at the crazy crap he accomplished during that time killing giant wild animals, taking out a penis-faced pirate solo and so forth.
And Sabo was right behind that.
Oda included that intentionally so that we could see if Sabo pursued his path of fighting and getting stronger, he'd be capable of great things.
So when he shows up again we're not all like, "Whaaaat? Bullshit! He's no Ace!" Well no, he isn't, but he's damn close.