Kate Hudson ''once'' slept with a co-star.

The 'Fool's Gold' actress - who is expecting her first child with boyfriend Danny Fujikawa and has sons Ryder, 14, and Bingham, six, with ex-husband Chris Robinson and former fiance Matt Bellamy respectively - revealed the best advice she'd been given from her stepfather, Kurt Russell, and admitted she'd broken his rule on one occasion.

Speaking at the PopSugar Play/Ground event at the weekend, she said: ''My dad told me, 'Never sleep with your co-stars' and I listened, except once. Once.''

The 39-year-old actress told the crowd all she wants for her children is the same thing her mother, Goldie Hawn, wanted for her; happiness.

She said: ''At the end of the day, when you see kids struggle it's the hardest thing to see. I just want happiness for my kids.''

The 'Almost Famous' star thinks it is much more difficult for girls than boys thanks to the rise of social media.

She said: ''I see beautiful young girls and all of a sudden I look at them, and I'm like, 'Did their lips get five times bigger?' And I think, 'This poor thing, it's all about the image' ... for me, with my kids, I just want them to feel good.''

Though Kate thinks her choices of partner are ''important'' when it comes to raising her family, she's pleased just to know her boys are happy.

She said: ''It's important, but it's really not that important, because I got really happy kids.''

Meanwhile, the blonde beauty recently admitted she's found being pregnant with a daughter very different to when she was expecting her sons.

She said: ''There's all kind of differences. They say girls make you sicker and that, for me, has been the truth. It's been interesting so far.''