Damage to thymus in a child may lead to

The thymus is a primary lymphoid organ located near the heart, beneath the breastbone. It is large in children and degenerates with age.
Immature lymphocytes formed in the bone marrow migrate to the thymus, where they mature into T-lymphocytes under the influence of thymosins.
T-lymphocytes are responsible for cell mediated immunity (CMI), such as the rejection of grafts and the killing of virus-infected cells.
So damage to the thymus in a child leads to a deficiency of T-cells and hence to the loss of cell mediated immunity.
Antibody mediated immunity depends mainly on B-lymphocytes, which mature in the bone marrow; haemoglobin and stem cells are produced in the bone marrow.
So damage to the thymus may lead to the loss of cell mediated immunity.