The most likely explanation for a child exhibiting a heritable trait that neither parent exhibits is:
The trait is recessive, and the child inherited a recessive allele from each parent.
This means that both parents can be carriers of the recessive allele (have one copy of the recessive allele and one copy of the dominant allele), allowing the possibility for their child to inherit two copies of the recessive allele (one from each parent) and thus express the recessive trait.