What will those parents do at home?
I had two older brothers growing up. We shared on bathroom. We were never in the bathroom together. We were smart and sane enough to know to wait until the person occupying the bathroom was finished with it.
Will they only stay at hotels with double bathrooms attached to every room?
Since when do siblings have to be in the bathroom at the same time?
What is the problem with a double-size bathroom with a lot of separate cubicles?
If that's what you want, then YOU go use a gender neutral bathroom.