Have you heard the saying “little pitchers have big ears”? It’s an old expression used to warn that “children hear everything”. There are many examples of “what not to say”. However, basically, it is anything that is a put down of the other parent (or their family) – from the blatant to the less obvious. A child will pick up on the tone of voice used. Then they will know it’s a put down even if the words don’t suggest it.
Also, comparing them unfavourably to that parent – for instance, “you are just like your father” is a huge no no.
No one would be comfortable hearing bad things said about someone they love. A child needs to be able to freely love both parents and to make up their own mind about that parent’s behaviour.