As an EYFS teacher I don’t think this is reasonable at all. There will be lots of children with ALN/ SEN who don’t have a diagnosis or an IDP/ EHCP yet on starting school. In most cases if a child is not toilet trained by the time they start reception there will be some sort of additional/ medical need, even if it’s not known, or it will be a sign that they live in a dysfunctional or disadvantaged household where their needs haven’t been prioritised and parents have neglected to potty train them. This will only stand to make the experience of school, and the ability to attend school, more difficult for a set of already disadvantaged and vulnerable children.
It should also be added that in Wales children typically start school the term after they turn 3 and the article states this will apply to nursery aged children. Lots of typically developing children will not be fully reliably toilet trained on turning 3, again this is going to make school education less accessible for children who are at the later age to be toilet trained. It’s been shown that nursery attendance is beneficial for children but this will just widen the gap for those who aren’t reliably dry and don’t have parents who are willing or able to pick them up. For a child in an abusive or dysfunctional household who has an accident at school I can also imagine that there will be a negative attitude from parents having to go and pick them up and change them, maybe an increase in emotional or physical abuse towards the child and making the child feel shamed for something they cannot help. All this policy does is punish children.
I’m also not sure that this really reduces the need for staff, realistically if a child has had a soiling accident they would still need to be removed out of class, their parents would have to be called and somebody would have to wait with the child until the parents arrived. I don't see how this will be significantly easier for schools to manage than just having somebody take the 5 minutes needed to change the child in the first place. I teach many children with SEN who are not yet toilet trained and me and my TAs all know that changing a nappy as and when needed will be part of the job for that student, similarly some children aren’t reliably dry and cleaning up an accident and helping a child change clothes is expected. That’s EYFS! I’m surprised if there are early years teachers who don’t recognise that this is part of the job and would sooner see a child removed from class and from the learning environment in response to an accident.