When an Apple gift card redeem lands near "invalid code," the rest of the sentence is not the same each time. This page keeps those endings apart. It does not walk through a redeem. Not activated is not one of these sentences.
Is an Apple gift card invalid code the same as not activated?
No. Not activated is not in this error. Read the rest of the sentence beside invalid on the Apple gift card: a named storefront, a typo, and already redeemed are three different sentences. The storefront sentence is on what to do when an Apple gift card's region does not match. Do not file the whole sentence as not activated before you have read the rest of it.
Does a child account also say the Apple gift card code is invalid?
A child account is a different sentence and never reaches an Apple gift card redeem. Do not file that sentence under invalid code. The account type stops the redeem before the gift card is judged, and that is not the same sentence as a gift card that does not match.
The Apple gift card was already redeemed. Why is the balance still missing from the account?
When the balance is already in the Apple account and the page has not refreshed, it shows up after you sign out and sign in again. That is not a second redeem. Entering the same Apple gift card again still returns already redeemed.
A named storefront, a typo and already redeemed can all sit beside the word invalid. A child account is yet another sentence, and it never reaches redeem. What appears after a refresh is the balance already redeemed into the account.