I think it is incredibly difficult but I don't think you have to be positive if it isn't true.
I haven't managed it at times in all honesty, however Ex has been truly awful post separation so I haven't been able to hide the impact of his behaviour to dc. He is also angry with them so they often talk to me about him and ways they can cope.How he treated you will be how he treats the dc in the long term.
I think it does depend on the dc ages as mine are older so I can label his behaviour and let them know it's not ok to behave like that. I think the adage about labelling the behaviour rather than the person is a good approach.
Ex feigns poverty but the dc know that his latest toys are expensive so they realise he isn't telling the truth.
I tend to say positive stuff when it arises, such as your Dad is good at cycling, because it is true, but I can't say he is a good man because that isn't true.I also recognise good behaviours in others, so they have role models.
If the dc are old enough they will know their dad had an affair and that is not acceptable behaviour in any situation.
My dc seem to manage to know that they can love their dad but accept his behaviour is often not good which means he can be very unlikeable at times.
I would never be abusive to Ex, infront of dc or behind his back because I have higher standards and I think holding yourself to a higher standard than your Ex, will in the long run make you feel better.
Deadbeat dads will never have a good relationship with their children which is priceless so you will be better off in the long run.
2 years down the line my dc have worked out Ex for themselves, my youngest made a comment to a friend that surprised me as he is able to recognise poor behaviour without writing off the whole person, it is referred to as object constancy.