The saiyans tail is a recessive gene, which means both parents would have to pass the gene to the child for it to show, and gohan has a tail, so chi chi and goku wouldve had the trait to pass to gohan for him to have the trait. maybe goten and trunks dont have it cause chi chi didnt pass it to goten, and bulma is full human. maybe it wasnt poor planning on akira toriyama, but something people missed