Sibling vs Parent/Child relationship detection using whole-genome sequence data
With at least three samples, you can determine this by looking at the IBS0 output column (from either plink2 --make-king-table or KING): if the relationship in question has a very low IBS0 value compared to the other pairs, that's parent-child.  You can try to do the same thing with only two samples, but with no other IBS0 values to compare against it's easier to misjudge.