The children do need a routine. It should allow for flexibility but too much flexibility can lead to control or an attempt to control the other's life, as well as taking advantage of a situation.
Here's an example.
I had said ex could have D whenever he wanted during the week - just to encourage him to spend time with her.
It seemed all willy nilly, he couldn't be counted on so I asked for a set day during the week. Now I normally, generally, usually expect him to get her from my parents on Wednesdays. It takes some of the control back from him. And I can plan things around that day also.
....Understand, that I can't, not be what I am I'm not the milk, and Cheerios in your spoon ~ Avril Lavigne ~ ..."Nobody's Fool"...