In a video posted to her Youtube channel on Tuesday, the former Las Vegas star's 4-year-old son, Brooks, nearly spills the news, but gets excited when his mother opens a big box to reveal a bunch of blue balloons.
"We're going to have a baby brother," Brooks chimes in before singing the Batman theme song.
"Brooks is really excited to be having a baby boy," Sims says. "He's already named it Jack and the Beanstalk-Batman. That's his full name. It's been really sweet to see him so involved."
Sims' husband, Scott Stuber, is also excited about having another son. "When they first told my husband we were having a boy, I swear he almost started crying," she gushes. "It's happening. We're having a boy."
While at the Casamigos Tequila Halloween party a few weeks ago, Sims told ET that she already has some names in mind
. "If it’s a boy, I love Wesley and Finley and Dutch and Wilder," she shared.