If an employment contract has fixed hours written into it (normally there’s a clause to say there should be some flexibility which is reasonable and normal), then the employer needs to consult with you prior to making any change to the working hours. It sounds like there’s been no prior consultation and also that there’s some ‘prejudice’ about you having children/requiring childcare.
Therefore I would recommend you tell your employer in the first instance that you are entitled to be consulted with about the change of hours and that him/her enforcing a change constitutes a breach of contract which you could make a legal claim on.
If there’s really no chance after that of reaching agreement, then you may need to resign and explain your reasons for doing so – and say that you feel you’ve been discriminated against too on the basis of your gender/having children…. I don’t know if any other colleagues are being treated in a similar way – it doesn’t sound like a very professional set-up based on what you’ve told me! I would advise contacting ACAS or your local CAB for further advice if required.