I ex speaks bad about me to my elder two sons, I make a point of NEVER running him down to them. I tell them off if they are out of order to the point of putting the phone down on one because he felt it was acceptable to swear while talking to me on the phone ! I will not be held to ransom by my sons (one is 21 the other 20) for them to like me and it seems to pay off.

I would carry on regardless and just be a mum they only have one mum as as they grow older the respect will take over.

If anything make excuses for your ex ie Oh well its just his way etc.