Busy mom Jennifer Lopez is back in front of the cameras after a three-year absence from the big screen, and ET's Kevin Frazier got a chance to hang with the star on the L.A. set of her new movie from CBS Films, 'The Back-up Plan.' But where are twins Max and Emme? The twins, just a little over a year old, are always a few steps away from the set, and Jennifer says, "I bring the babies to work with me. I love it. Honestly, one of the best days of my life was the first day back working on this film -- I wanted to do this film really badly -- and bringing my babies with me that morning. I was like, 'Oh my God, they're with me, I'm making a movie, they're here!' It's great having kids on set. It's the best." Hubby Marc Anthony is also nearby to keep an eye on the tots for the night shoot at the Warner Bros. lot, as is the "Dog Whisperer" Cesar Millan, who has a cameo in the film and jokes that sometimes people want him to work on their kids! In 'The Back-up Plan,' Jennifer plays Zoe, a pet store owner who just can't find Mr. Right. Desperate to be a mother and tired of waiting, she decides to go it alone and get artificial insemination, only to meet Stan ("Moonlight" star Alex O'Loughlin) -- a man with real possibilities -- the same day. Jennifer is in amazing shape for the film, and she explains that it wasn't a piece of cake. "It did take a lot of work," she admits. "I thought it was gonna drop off easily because I had been in shape my whole life, but it wasn't. I gained about 50 pounds with my twins, and the first 30 dropped off like that, and I was like, 'Ha, this is gonna be so easy.' That last 20 -- that took a while."