It is possible to get the Golden Scale without a 15 pound fish, and even to do it as Child Link. Go to the fishing pond and pay to play. Catch a fish that will tie or break the current record (it starts at 6 lbs). Now go swimming in the pond, so that you are in the deep water and actually swimming. Face the manager, hold Z and swim towards him. Without letting go of Z, talk to the manager and tell him to weigh your fish. If it ties or breaks the current record, you will get the Golden Scale. This is a bit faster than getting the Silver Scale as you are then right next to Ruto's Bottle, and do not have to scale Zora's River either.