This is deplorable!!!! how dare they suggest that your child returns to nappies simply because they cant get their arses in gear!!
I am a nursery manager and if one of my staff members said this to a parent or behaved in that way i would haul them over the hot coals!
Firstly, all children should be offered and encouraged a toilet break at least every hour, they can be taken in groups so that one person doesnot spend their whole day out of a room, children should be taken to toilet or have their nappy changed as and when needed, simply leaving a child to soil themselves or in a dirty nappy is disgusting.
Surely its quicker to whip them to the toilet than to have to wipe them down and change their whole clothing?
Your poor lo is obviously being affected by this - speak to the manager immediately and take action.
I would not recommend going to the play group to take her to the toilet, a parent coming and going throughout the session will confuse and upset the routine and children. I would however ask the manager to provide a thorough copy of the daily routine including toiletting and meals / snacks and staff numbers throughout the period.
This is disgusting behaviour!!!! they are affectively stunting her development and self care skills in even sugesting this. It is their duty of care to observe their own practises and ammend them should they they not be appropriate for the children, good luck to you