Sorry, Barrybran, siding with the W on this one. Discuss things with her before bringing them up to the kids. You told S you had to discuss it with W first, and now if it doesn't happen, she's the bad guy. Not cool.
My H currently has our D12 5 days every two weeks. Recently, I offered him another day, to try to get the percentage closer to 50/50. He surprised me and said no. Thank goodness I hadn't said anything to D12, can you imagine how rejected she'd feel?