You can write an email to the nursery. Tell them its not acceptable. I used to work in nursery for many years we never let any kids go home dirty our policy was if a child gets messy and dirty get them change before home time . If still doesn’t work u can report them to ofsted because it’s under neglect…
I would take a picture and send them an email every time it happens. Hopefully if you are consistent they will start taking it seriously
Hey, for the cost of nursery these days id be sending an email to have something in writing on record. It's basic hygiene, shouldn't be happening.