I would consider the language used in the passage as a useful clue for getting started at crafting your argument (Scout as adult, or Scout as a child). Right away, the statement "and it made me sad," sticks out to me as something a child would say. You might also think about how children commonly see material objects (like the items in the tree) as holding more value than moral or ethical virtues, gifts, or intangible qualities that constitute a "gift" from the perspective of someone older and more mature. So, if you wanted to make the claim that Scout is commenting as a child, you could use language as an indicator and build your argument from there.
This is one direction to take, but I would challenge you to think about the possibility that there are strong arguments for both sides. We (as readers) know that the narrator is telling the story as an adult, and this passage can also speak to Scout's ultimate understanding of what it means to give or receive something you cannot put a price on: love, forgiveness, kindness, etc. Think about the statement, "we never put back into the tree what we took out of it." Perhaps this is the voice of the adult Scout speaking into gratitude for what Boo did for them. I think you can write a critical essay that explores both perspectives instead of choosing one or the other, but either way you go, you should have enough to support your argument.