The so-called recessive inheritance refers to the recessive gene in the allele that causes the disease (such as aa), that is, the child will get sick only when the gene is Aa, and will not get sick when the gene is Aa, but he carries this disease-causing gene. If both parents have normal phenotypes and the children are abnormal, then both parents must be aa and the children are Aa.
Assuming that the father is sick, the mother is not sick, and the child is sick, then the father's gene must be aa, the mother is aa, and the child is Aa.
In fact, as long as the genotype of the disease is definitely aa, then the arrangement and combination of parents must meet the requirements of forming this aa combination.