Ex is poisoning well with new boyfriend

So my kids came back last week talking about how "mom's boyfriend is way cooler than dad" and how he took them to Topgolf. Cool for them, genuinely. But then my son mentioned he coaches youth baseball and the dude is literally inserting himself into EVERYTHING. Now my ex is talking about having him there for Thanksgiving at "her place" like he's already part of the family. My kids are 8 and 10. I'm trying not to be petty but... does anyone else deal with this? How do you not let it get to you?