I would call the nursery and ask for the name of the owners... then write to them, with a copy of the letter to the manager of the the nursery. Explain the situation and how shocked you were at the way that Fynn was treated and how distressed he was when you arrived to pick him up. How that surely this kind of treatment was 'child neglet' and that you are looking for answers before you take the matter further. Give them a time limit to reply - say 10 days.
THEN go to ofsted.. whatever they say in a letter, (although of course they may not choose to reply) this way you will have so much ammunition. It might be a good idea to write down what happened yesterday and today, with time and names etc while it is still fresh in your mind. Good luck
to both you and Fynn.. and so glad you have found a nursery that you feel happy with... hope Fynn enjoys it on Thursday