How do you handle it when your ex badmouths you to the kids?

My 8 year old came home and said mom told him I didn't want to spend time with him because I'm always working. That's completely unfair and not even true. I've been busting my ass to see them as much as I can within our agreement. I don't want to go nuclear and bad mouth her back but I also don't want my kids thinking I don't care. Do I talk to her about it privately? Do I address it with my son? This is making me angrier than the actual custody fight did.