I went back after mat leave to 0.6 and had a pretty good reduction in teaching - we see everyone else's workload and I know my admin job was smaller than everyone else's too. Now I'm on 0.8 (4 days a week).
Your HoD sounds good, mine has been cr*p (old one - new one is much better) and I had a lot of support from my union to allow no teaching after 5 (as nursery shuts at 5.30...)
I am ruthless though! I have my working days on my outgoing email signature and I tell all my students not to expect replies on my non-working days. They are much better than colleagues! Internal colleagues aren't too bad but external - especially overseas - colleagues cannot comprehend AT ALL. It doesn't help that I have a good number of collaborators who either have much older children or who live in countries where there is hot and cold running childcare.
I work evenings if I have something easy (marking etc.) or urgent (haven't quite finished something at 5.10 when I have to pick up DCs). I can't work weekends unless I'm away from home as the DCs are small and just relentless.
I posted on here about school aged DCs and part time working - I really don't want my DCs to be in after school care every single day all the time - for a start they won't get to do any activities. There was someone else who replied who worked till 2pm 2 or 3 days a week which I may think about when both my DCs are in school. DH currently also works 0.8 and I think my department would agree as long as I could occasionally swap school pickups with DH.
Before DC1 started school I was struggling with flexibility as I had no childcare, ever, on my day(s) off because the campus nursery never offers extra days. My dept weren't happy with that but it's a bit easier now because a) DC1 is mainly in school on my day off and b) DH can occasionally swap days off - so DC2 doesn't have to go in for extra days either.
I was reasonably senior when we had the DCs and I have also started caring a lot less... resting on my laurels and have no major desire to be promoted very soon, and making the most of my advisory role on people's projects to seem important while doing not THAT much on any given project (e.g. have just applied for some funding which will almost all be undertaken overseas so if we get it, I'll waft in to see them a few times a year but carry on working 0.8 here).