The celebrity couple is parents to eight-year-old Stellan and baby daughter Agnes, while the actress is also mother to 14-year-old son Kai from a previous relationship.
Both stars agreed that one of them should always take time off when the other lands a job, but the chance to film financial drama Margin Call prompted Bettany to break the promise while his wife was away on a shoot in Michigan.
As a result, Stellan and Kai - as well as the family's dog - found themselves sleeping on set as the cameras rolled.
Speaking on Britain's The One Show, Bettany says, "We try as best as we can to work one-on one-off, it's really simple and it hardly ever works. It didn't work (on Margin Call). It just came up, it was a great script, she was working in Michigan. I read the script, I met the director, I really wanted to do it - it had Kevin Spacey and Stanley Tucci and I said please let me do it...
"There were night shoots, so consequently I had my kids on set with me. There were no trailers - it was a tiny movie but I had this little office space on the trading floor we were shooting on and I had an inflatable mattress in there with my kids asleep and my dog."