Where a ground for the referral of a child's case to a children's hearing under the [1968 c. 49.] Social Work (Scotland) Act 1968 is that mentioned in section 32(2)(g) of that Act (commission by the child of an offence) and that ground has either been accepted by the child and, where necessary, by his parent or been established to the satisfaction of the sheriff under section 42 of that Act, the acceptance or establishment of that ground shall be treated for the purposes of this Act (but not otherwise) as a conviction, and any disposal of the case thereafter by a children's hearing shall be treated for those purposes as a sentence; and references in this Act to a person's being charged with or prosecuted for an offence shall be construed accordingly.