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.
In this scenario, both parents can be carriers of the recessive allele (having one dominant and one recessive allele), but do not express the trait themselves because they have one dominant allele that masks the effect of the recessive allele. When the child inherits the recessive allele from both parents, they express the trait.