What's a gentle way to tell my kids that I will be spending Father's Day with my girlfriend?

My GF has a really nice barbecue set up and she promised she won't let her kids buy me any ugly ties. My wife always gets these ugly ties and lets the kids wrap them up like it's really cute. Who wants a drawer full of ugly ties? She never got me that new grill I wanted even after I said I would watch the kids while she worked some overtime to get the cash together. Her priorities are really messed up and it makes me sad that I wasted so much time with her.