Next Nursery Day, give them a (fairly formal) letter, saying something like :-
Dear Nursery Manager,
As you will know from the concerns I have verbally addressed to your staff, I continue to find that my two year old child is not only spending extended periods of time outside inappropriately clothed, but is also spending much of that time alone and unsupervised.
First of all may I say that, on the plus side, I applaud and support your fresh air policy and am delighted that my child joins in with all forms of outdoor weather and age appropriate play.
I will ensure that I always provide suitable clothing for this and think you will agree that in the winter (and for the level of mud-play that seems to be enjoyed) an all-in-one outdoor suit, wellington boots, a hat and gloves would be suitable.
These will be provided daily and I ask that, for your part, you ensure that all staff are aware that they must ensure he is wearing this outdoor clothing for outdoor play.
Further to outdoor play, and of particular concern, is the fact that he is spending a lot of his outdoor play-time alone. That is, entirely alone outside, so not supervised by any member of staff.
At just two years old, this does not seem appropriate (or even allowable) from a safeguarding point of view. Can you advise me of your policies with regards to supervision outside?
I do hope these matters can be addressed with immediate effect, and will review the situation within the next two weeks. I look forward to hearing from you,
Yours sincerely,
O8O818