Agree with Ribena, if there is a genuine business need, they need to reduce headcount and costs and as part of that they need these roles to be flexible as they've described, then they can do it, and it's not discriminatory if you are all women.
Have they explained to you any further why the changes to the roles are necessary? If a company needs to save money then that obviously explains the need to reduce the number of posts but it's not necessarily as clear cut why the need for flexibility. Presumably it's about covering the same amount of time with less people.
If you can see that the new requirements for the role are necessary, it seems your options might be to either take redundancy and look for something else, or alternatively explore your childcare options. I don't know how much information you've got or what your normal working arrangements are, but it could be possible to work early mornings and evenings with a DP who is away some of the time, depending on how much notice you have, how much choice you have about which days they are, whether there is any flexibility with your DP's job, whether you have anyone else who can lend a hand with childcare on the odd occasion.
Obviously it would be a lot harder for you than for your colleagues, which is why it feels so unfair, and it is unfair, but it might not be impossible.
Something else you could think about. If you have DC under 6, you can apply for flexible working, which isn't necessarily about reducing hours, it can also be about arranging your shifts to better suit your family. You could apply for one of the jobs and then once you've got it, put in a flexible working application to at least fix the shifts to make it easier for you.
I'm trying to give you a few things to consider. It seems a bit hopeless now, but I'd say apply for the jobs, you aren't losing anything by doing so, unless they are offering a better redundancy package to those who go voluntarily, which they probably aren't. Apply for the jobs, then explore your options a bit further down the line.