Eh, I think stats don't really help for the children? Sure, they got some tidsy-bitsy boost of starting stats, but it wasn't that significant? For children, since you can't alter Leaf and Altenna's skills, probably you can do is to make sure to have both Cuan and Ethlin stocked with necessary weapons to pass down to the kids (at the very least, Light Sword for Leaf) before Chapter 3 ends.
Spamming with Return Staff is the quickest way to spam exp for staff user. I think there's one chapter that will require you to teleport the majority of characters to base castle - I got my Aideen from level 10 to whopping level 20ish in this one chapter.
Jamka is easily one of the broken character early in the game simply because of his Killer Bow. Lex too, most of the times, because of the Hero Axe and huge defense. As I wrote above, sometimes changing weapons in Arena from stronger weapon to a lighter weapon does help. This is how I clear arena with Fury - initially she's a bit weak, even with Hero Lance (at least in my game), so sometimes I switch her weapon with Slim Lance that has fewer Might but much lighter that she survives most of the arena by dodging.