Sorry for your situation it’s got to be tough.
What’s your relationship with your ex like? The best opinion I think I can give is to talk. Either to the kids and explain to them your point of view, and / or your ex. You may not be together but you’re raising the kids together and for me the kids shouldn’t be weaponised. Don’t have a hostile discussion though.
Essentially how can your kids / ex know there’s something wrong if you don’t tell them?