There is an impressive but purely compensatory growth response of the child's kidney transplanted into the uremic adult, but no hypertrophy or hypotrophy of the adult's kidney in the uremic child.
Two separate types of renal growth seem to occur. Compensatory hypertrophic growth is a rapid reversible response to a functional nephron deficit. Obligatory growth occurs as one grows up, and is not reversible.
(JAMA 228:1143-1145, 1974)
Sherman J. Silber. Renal Transplantation Between Adults and ChildrenDifferences in Renal Growth. JAMA. 1974;228(9):1143–1145. doi:10.1001/jama.1974.03230340045031