Oh My Goodness! OP I am go smacked that this has happened to you. Your poor DD, and poor you and your DP.
I agree that you should write everything down. The facts, with rough timings if you can. What time you arrived, who you called and in what order and as much as you can remember with the conversations you had with the staff from the school next door.
This is certainly a matter for Ofstead, do they have an out of hours number you can call this evening? Try calling them on any number this evening as this is a matter that can't wait.
I would also call the police 101 number. Log what has happened and they may also be able to advise you who else you need to contact.
Your Local Education Authority also need to know about this ASAP. And I would give the LADO a call. Yes the out of hours number is for children in immediate danger, however I would argue that all children in the 'care' care of the nursery tomorrow are in potential danger.
Call as many numbers as you can find for the authorities as this is an urgent matter. Out of hours people may ask you to call back in the morning, but they should log that you have called.
Huge hugs to you. If you can you and DP should try and write a statement each without conferring. And sign, time and date it.
I work in a secondary school and we have to write similar statements for any incidents that happen (usually student fights, but the statement principle is the same).
I would discourage going to the press about this until you have had initial meetings with the authorities. However I do feel that the story should be told (perhaps wait a few days though)
My thoughts are with you. I am sure that there will be a full scale investigation into this.
Have a glass of vino and chill this evening xXx