I work for a large Co too.
I start mat leave ending this week, I currently work 5 days fulltime & have a pretty full on role, as the Co. runs 24/7, I advised them I wanted to return to a nopn important role, with no stress & also of evenings when my hubby would be home.

My big boss tried to negotiate with me to stay on as we get along well & my role is important but I told him my kids are my priority & having 2 littlies at home it doesnt justify my hubby working (as I earn more) so I said I am soprry but this wont work for me so come back to me with something that will.
As I knew he wanted me to stay, so he has come back & I have agreed (happily) to work from 8pm til midnight 5 nights a week, in a no responsibility role as a Level A worker (which gives me a higher pay rate) but come midnight I leave my work behind, where as my current role is also on call...
I can now be home with the 2 littlies have dinner cooked etc & we all get to eat together, bath & put littlies to bed & then I head off to work for 4 hours which will be a breeze compared to my current role & all for a similar pay as it's nights & has penalty rates...

So definately discuss it with them sooner than later!

I am very lucky that where I work I get along with the big boss & he has said to come to him when I am ready to move back to Full Time in however many years time!!!!