The Bible says in Proverbs 11, "Whoever belittles his neighbor lacks sense, but a man of understanding remains silent." The co-parents I've met who impressed me lived by this verse. Though they sometimes were tempted to say negative things about their ex or to their child about their extended family they chose to say nothing at all. They understand that negativity invites negativity and that children suffer when parents belittle each other. If you can't say anything nice, hold your tongue.