What do you think about me telling my partner that it is his fault and not his ex that he lost relationship with his kids?
What makes you think it's his fault?
He's doing nothing to fight for his lost relationship. He doesn't even know the reason why the ex left him.
If any situation doesn't involve you, instead of getting in the middle or putting your two cents in, you should mind your own business. Let your partner sort things out with thier ex.
Well I have said my piece of mind and now leaving it to them.
That's fine, but you really need to leave it to them now. It is between your partner, his ex, and their child on what happens in that relationship. Sometimes it is better to just stay out of certain things.