"It was not customary for the Levites to enter upon their peculiar services until they were twenty-five years of age, but Samuel had been an exception to this rule. Every year saw more important trusts committed to him; and while he was yet a child, a linen ephod was placed upon him as a token of his consecration to the work of the sanctuary. Young as he was when brought to minister in the tabernacle, Samuel had even then duties to perform in the service of God, according to his capacity. These were at first very humble, and not always pleasant; but they were performed to the best of his ability, and with a willing heart. His religion was carried into every duty of life. He regarded himself as God's servant, and his work as God's work. His efforts were accepted, because they were prompted by love to God and a sincere desire to do his will. It was thus that Samuel became a co-worker with the Lord of heaven and earth. And God fitted him to accomplish a great work for Israel. CE 216.3"
"But Samuel ministered before the Lord, being a child, girded with a linen ephod."
The EGW passage directly references and expands upon the biblical account of Samuel ministering as a child, specifically mentioning the linen ephod.