I think you need to have an open and honest conversation with your manager:
Here are my objectives.... here is what I have achieved... I am meeting all of my goals.
Here are my agreed work hours - I do these hours between X and Y... I take my one hour break between A and B. Are you OK with that? Is HR OK with that?
Does my break between 5pm and 6pm impact on the business? Is this something that we can work around together?
I am reliable and committed to my job and to the business. If there are issues, I hope that we can work through them together.
I wonder what has been agreed by your manage and what has been said by her? You seem to imply that she has agreed that you will take your break at the time that you do, but also that you are not available for work at that time - try to get that cleared up.
I know that the reason you have to have your break at that time is because of your child, but try to frame it around your work, when you have this discussion... Objective, targets, priorities, time spent working....
(I do know how difficult it is.. I worked in a team where I was responsible for the biggest projects, the biggest customers, the largest budget area, I delivered everything ahead of time and under budget, all of my 'customers', internally and externally were happy with my work and my team shifted me sideways because "no one" wanted a part timer on their team - 'no one' was one person! I took an internal move and they loved me.... they were grateful for my work - I did work in the evenings and at weekends, occasionally, when required, but mostly I worked my hours alternating short and long days, 10-4 and 8-6 and Fridays off - but I did have a partner also working full time who did short days on the days that I was in the office before 8 and after 6.).