I know what you're thinking, because I thought it, too: John Stamos brought the kid to see Rebecca Romijn-Stamos on the set of an X-Men movie. But not only is that not Rebecca Romijn-Stamos, that's not even John Stamos.
It's other blue model Stella Tennant, wearing random Italian couture in a Steven Meisel photoshoot for Italian Vogue, March 1999, more than a year before there even was an X-Men movie.
Anyway, I can't tell if that's actually Tennant's kid and baby daddy, or if they're just props, but this totally random umbrella stroller sure makes me marvel at the sad state of It Strollers before the Bugaboo came along. Truly it was a different age.
Steven Meisel | Stella Tennant | Vogue Italia Mar 1999 [tumblr thanks dt reader erica]
Also the Romijn-Stamoses didn't even have kids [wikipedia]