Why is gold placed around the child at a pidyon haben?

Hello.

1. The reason the firstborn is decorated at the time of the redemption with ornaments of silver and gold is because of the beautification and cherished nature of the mitzvah (see in the book Pidyon HaBen KeHalachah, chapter 8, section 5, and the note there).

2. One of the reasons for this custom is in order to distribute something from the meal to all those who were unable to participate in this meal, whose importance is very great. Therefore, so as not to trouble the one hosting the pidyon, it was instituted to distribute garlic, which is something pungent that is fit for a meal to be eaten with it, and a small amount of it is enough, and it will easily suffice for all the participants and their household members (see there, section 40, and the note there בשם ספר לקוטי פנחס).

With blessings,

Hillel Meirs