My Mum is lovely, but my Dad is odd, tends to be all or nothing with the children, DD 8 and DS 13.
We don't see all that much of them and I have reduced visits because my Dad is getting odder (very recent) and had been quite inappropriate with DS especially. Using language and saying things that are quite 
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But the DC's at the moment still love seeing them when they do.
My MIL (Nana) . 
. I have found very hard work, she often makes DD cry, mocking her, she winds them up and also, more importantly, (despite the break up of mine and DC's Dad's relationship being as a result of EA, and things being very complicated with DS, due to his messy relationship with drink), has consistantly brought him Alcohol and made a point of giving it to them in front of the DC's 
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The kids hate going to see her and a visit was arranged this weekend which initially they agreed to when talking to their Dad, but voiced to me that they really don't want to go. I told their Dad and he was annoyed, but didn't insist.
It has just come up and Ex resents that the kids will see my parents but don't like seeing his Mum. He raised the fact that DS used to complain that He and my Dad used to talk a lot (to the exclusion of almost any of us) when we all used to visit to the point that my Dad rarely interacted with the DC's. He asked why they will see my folks after this but not his Mum.
I wouldn't see my Dad at all if it weren't for my Mum, he as an abusive, manipulative, control freak which as of yet the DC's haven't really seen. It is directed towards women . I have already limited the DC's contact with him because of his unpredictability.
My thoughts are that they should have the choice to a great extent. And that the experience of GP's should mostly be pleasurable and fun, and insisting that they see someone who has consistantly made them unhappy is just wrong.
I don't influence the DC's re their Nana, I believe it is their choice, and despite them wanting to see my parents at the moment, if my Dad continues to deteriorate further, I would reduce their contact to keep it as positive as possible.
This came off the back of another 'heated discussion' where Ex had decided when he wanted to see the Kids this week as he has time off and I suggested an alternative (I am still working this week). He said 'no', when I questioned why he simply said 'because I said not'. I said I thought these things should be dealt with through negotiation and compromise so he walked away from me.
Actually I am a bit pissed off. Needed a rant.
He said IABU and he is not being given choices. I asked him to tell me what he meant because if I am being unfair, I need to address that, but he wouldn't, but said 'things were stacking up'. When I suggested we used a baby sitter to try and talk properly he said he didn't want to.