First of all you are doing a great job, it is incredibly hard to be a working mum I found it really hard and stressful working part time even when I knew my dd's were perfectly happy and being cared for by their dad!!!
I think the idea to have a review meeting is a good one.
have you checked out that you are getting all the benefits you are entitled to ie tax credit, housing benefits, council tax benefit, Gingerbread are really good with that sort of thing. I know they have a helpline number and usually have up to date information, you could perhaps ask them to help you work out your finances if you dropped your hours.
The other thing I would suggest is to contact ACAS they are again a brilliant source of information about employment issues, it does seem like you are being bullied. I dont understand why they are taking notes about the time you arrive, especially when they agreed with it when they employed you. You do have a right to ask for flexible working hours if you have a child under 5, although they dont have to give it to you.
Have you asked the nursery staff how your dd is after you have left, it may well be that she is playing happily after a few minutes, if you knew that it would be a huge worry and upset off your mind
I hope things improve for you