Hardly anyone finds public toilets a pleasant place when nature calls.
But some kids (and adults!) react very poorly to the sounds of the public toilet. It might take some experience or some discussion to figure out that the sounds of the public toilet, specifically hand dryers and flushing, are what make them the most unhappy. One way is to accompany them into a private-but-public bathroom and make both sounds. If they are significantly distressed in that situation, but using the same bathroom without both of those sounds (you will have to flush after they leave and bring a napkin for hand wiping) then you might be able to avoid discussion.
This is really easy to understand if you do auditory treatments such as Therapeutic Listening. The low frequencies of both events can be disorienting or even frightening for people of any age who struggle with spatial processing. The louder and closer to the person these sounds are, the more they distort information.
Imagine suddenly becoming dizzy or suddenly having your feet go numb. Yeah, that would be distressing. Well, people with poor spatial processing are constantly using their eyes and movement to support their sense of where and when and how safe they are in a space.
Remove access to the instructive sounds and replace them with low frequency sounds, and they can feel anything from being “off” to being unsafe. Add in a history of trauma or hearing loss due to ear infections, and you have someone who might prefer to have an accident than use the public toilet. Or hold it too long and develop a “lazy bladder”. This is risky for everyone, but particularly for kids with low muscle tone Teach Kids With Ehlers-Danlos Syndrome Or Low Tone: Don’t Hold It In!.
Treating spatial processing effectively is usually a combination of vestibular activities and auditory activities to improve processing and increase tolerance.