Answer
In children who develops multiple tumors in both eyes, every retinal cell had a defective copy of the $Rb$ gene at birth.
Early in the child's life, several cells independently underwent a second mutation that damaged the one good $Rb$ allele, producing a tumor.
A child who develops a single tumor had, at birth, two good copies of the $Rb$ alleles in one cell (extremely rare) which caused a single tumor.
Work Step by Step
Some children with Retinoblastoma develop multiple tumors of the retina in both eyes, whereas others have a single tumor in only one eye.
