My guest bath is for guests, so it would not be an issue. That said, parents do not enter my home without being invited in. Drop-offs and pick-ups are on the covered front porch. Few parents come in on a regular basis for anything other than conferences.
Walking into homes without being invited is not a cultural norm, here.
Now, once they are invited in, custom mandates I offer them a glass of sweet tea, soooo...
Now, we are back to the guest bathroom.