Having read a bit more I wanted to add that having worked on cases where kids are abused and neglected etc, it really varies how much they tell the other parent when there’s shared care. It might be hard to believe the Dad had no idea they were that cold but the Mum may have said don’t tell your Dad or he’ll be sad, they might have just been quiet kids who didn’t say much. If the parents are on good terms as well IME they don’t ask kids ‘how was your time at other parent’s house?’ etc, generalising here but that’s usually/often where the parents are on bad terms or one doesn’t trust the other parent. If the Dad had no reason to question Mum’s parenting he might have just not asked and the kids feasibly may not have volunteered that information either.
Even if the kids had told Dad they were cold then honestly without living there himself there’s not much he can do about it. He could have spoken to Mum about it and she said the kids were making it up, she could’ve become angry that he was questioning her parenting and refused to discuss it, he could’ve offered to pay heating and she spent the money on something else. Shared care and responsibility can only extend so far.
Anyway this is a very long winded way of saying I don’t think it’s fair to apportion any blame to Dad here based on what is currently known.I imagine Dad is completely devastated and will spend the rest of his life either beating himself up because he ‘should have known’ or that he didn’t do more if he did know. I really feel for him and all other members of their family.