Put it near the end of an unboxing, after Zelda Item Get marks the reveal and before Kids Cheering lifts the celebration. That order keeps the thanks as the bridge between discovery and applause, and it also works after a quieter present when a bigger reaction would seem out of place.
About this sound
This is a short spoken thank-you from someone who has just received gifts, delivered in a clear, friendly voice at a natural, unhurried pace. It feels sincere and lightly cheerful, like a genuine smile after a surprise, rather than a big performance. Lasting just over four seconds, it fits neatly after a reveal without stealing attention from the gift itself.