I don't know if this is helpful, but I've been paid 80% for the last 20 years to allow me to have Wednesday off to help me to cope with being disabled. I have always done a full week of work in terms of activity and attainment, but I've had to accept the reduction. To be fair, I thought it was absolutely fair enough, so it didn't cause me any resentment at all at the time. Now people are being paid 100% to work four days a week and the rationale is that all do is they cut out is their wasted time - chatting, lunch breaks, popping out for coffee etc. I cut that all out 20 years ago in order to get my work done in the reduced time. I can't remember the last time I had a lunch break. I have been doing a full-time job for 80% pay for a very long time. I sometimes have to remind myself to not feel resentment!
With that in mind, I would be tempted, if I were you, to say "let's see what I can do during my probation - you are paying me to perform a role, not to clock in and out at a particular time, and I think there is a very good chance I will perform the role just fine in the time I work. If you aren't happy, then we can revise the contract". Mention these people being paid full-time to work a 4 day week, and tell them that as a mother, you are particularly good at making the most of your effort. It is worth a try.
In the end you may just have to do what I have had to do - accept that you got the thin end of the wedge. But, at least if they stop paying you at 3:30pm, by their system of watching the hours instead of concentrating on the role, you can stop thinking about work at that exact point, and tell them to bugger off if they want you to do anything later. It takes two to tango. But you can't be buggered both ways.