DO NOT IGNORE.
Do not mistake this as a simple case of the father being a twat and you needing to do what our society expects from "nice women", which is to rise above it and ignore.
This is not a casual act - "what shall we do tonight, kids? McDonalds or talk about and write down all the reasons why your mummy is a bad lady?!"
This is a deliberate, premeditated attempt to derail your relationship with your children, so please don't underestimate its potential.
Also consider that this may be the first you've heard of it but how long has it been going on?
This sounds dramatic but in a potential future Family Court battle to reduce your custody this would be presented by him as my "my children told me they were unhappy with ex wife's parenting of them, I was concerned and tried to advocate for them but she just refused to engage/attacked my parenting style".
Please never think that your kid's dad wouldn't do this to you. Dont think "he doesn't want them full time, so that's not what hes up to". It's amazing the amount of time and effort a useless or seemingly amicable ex will put into destroying your relationship with your kids to punish you. The primary aim of this is to punish you, not take on his kid's laundry.
Chat with your kids about how this came about and please please keep a record of what they say, dates etc. If you see this is part of an ongoing pattern then spend the money to consult with a specialist lawyer for advice on Coercive Control, which is now a criminal offence and can (and should be) reported to police. This can be easily online if it comes to it. This will also show that you were concerned enough to act. If you report online, the police will speak to you and importantly it will be logged.
Don't assume that just because your kids seem happy to visit your ex that they are not being groomed, they won't understand what's going on and your ex will disguise his motives. Don't try to fathom "why would he do this?" as just because you don't understand why, doesn't mean he's not doing it. This is not the time to "rise above and do nothing". Please be vigil for the sake of your family.