What to say to your children when narc tries his games on them??

I know the current body of thinking is not to bad mouth the other parent which I wholly agree with in respects of dealing with a 'normal' parent but when your ex narc uses his contact to insiduously imply that mum is a 'nasty' person and your children call you on it what can you say to reduce the damage?
Latest example being the narcs mother sent hand knitted jumpers to my children (we do not speak as she enables his behaviour and may be a narc herself)and upon opening I had said 'although nice I didnt like woollen jumpers' because I would probably shrink them and knew they would then construe this as sabotage! Needless to say my youngest child aged 6 innocently said to his dad, the narc, yesterday that I didnt like the jumpers..this then resulted in the narc saying how ungrateful I was, phoning their gran and lamenting how hurt their gran would be by me saying that. The boys then came home and said I was bad for saying I didnt like the jumpers?! Stupid I know and I realise I shouldnt have said anything other than they were nice now but it was an innocent comment.....apart from saying anything other than nice things about these people how do you neutralise situations such as this? It has left me spinning that the people who have tried to destroy me can gaslight me to my kids like this and I cant say anything in my defense as that would be involving the children in adult issues..so what do you do?