crush I'm not quite sure what you mean about what are your rights?
If you are being asked to do more than your contracted hours and you can't, of course you can say no.
I'm not sure what you mean by needing 'pre-warning' of things or not liking being asked to your face, I'm a bit confused. Unless I'm not understanding you rightly then there's nothing wrong with asking you things to your face or without warning particularly - maybe the issue is more how you are being asked or something, the tone of voice or you feel there is an expectation you should do it or similar?
Is this your line manager saying these things or a colleague? I think you need to ask to meet with your boss, just say you are feeling pressured to increase your hours, which you are not able to do. You don't feel your childcare arrangements are the concern of anyone else at work. You are fortunate enough that your mother has very kindly offered to take your baby for a couple of mornings a week, but you certainly don't feel you can abuse that kindness by asking for more help and you are very happy on the hours you are currently doing.
Similar with the illness issue, you need to say that you feel you should have been informed previously as there is a possibility that your DD contracted it through your contact with an ill child at work. Someone else might have some advice about this, working in a classroom, pupil illness and your own children being infected, I'm sure it's very common and someone else will know normal procedure.
Hope that helps, try not to feel down, I think when you have that conversation, you will feel much more positive, as you will have taken back some control which it sounds like you feel you don't have at the moment.