Tony doesn't begrudge anyone else their enjoyment of the holiday season; he just wants to be left out of it. He's never really seen the point. Bucky's usually all for it, but a disaster at work that forced him to cancel his plans to travel with his family have left him feeling a little curmudgeonly. The two meet and bond over their shared lack of interest in the holidays.
When Tony unexpectedly finds himself taking care of a pair of kids who rather emphatically expect Christmas to beĀ Done Right, he finds himself turning to Bucky for help.
Bucky's used to performing for kids. Prefers it, actually--adults can be really dismissive (or worse, totally into it and grabby). Still, when Natasha tells him he's helping her perform at the summer Maria Stark Foundation gala, he doesn't say no.