I haven't been on Mumsnet for years, but I've logged back in for some advice.
Very long story short- my ex husband currently sees our children (6 and 8) every other weekend.
The children always come home distressed about their treatment there (emotional stuff, being treated unfairly, eating junk food and feeling sick, being told they can't speak about me etc) and SS have had a little bit of involvement but said it isn't bad enough.. 
Ex husband has now suggested that instead of trying to make changes to ensure the children are happy to visit, he's proposing that the visits just reduce to once a month. This would destroy the children, they love him deeply, and want to spend time with him, but it's all so complex for their tiny brains.
I don't think this is in the children's best interests (Ex Husband concedes that he still wants to see them every other weekend but doesn't want to make them sad), my suggestion of just working on the issues to ensure they feel safe and secure etc is 'too hard'.
What do I do? He's not an awful person, just very clueless about children.
I fee like I'm after some articles about how changing behaviour and still seeing them often is far better than running away, and the issues will still be there when they visit... but this message is too long for google.
I'd appreciate some advice!