According to the information they had just obtained, an Elemental Child is a title or name one obtains after one's Elemental Affinity reaches an extremely high level. After getting the title, they receive many perks like reduced energy consumption when casting spells, increased affinity with Elemental Spirits of the same Element, quicker comprehension of Spells, etc.
In some rare cases, Spells of the same Element do less harm to the Elemental Child and could even cause a backlash or worse, being controlled by the Elemental Child but such cases are extremely rare.
And the reason why the dark barrier was being attacked was because the Dark Creatures had actually captured and caged the Elemental Child which the Elements didn't take kindly to. And as Evan increased his Affinity, the stronger the attacks became.
