Firstly, she probably thought she'd be kicked off the show alot sooner, so could have only been away for a week.
My friends fella works on an oil rig, is away a good few weeks at a time an dgets to call home very rarely. At least Cerys' kids were able to write to her and see her on tv.
It is a job, and some jobs take peopel away from home. It is unlikely to be of a huge problem to the children, especially if they're used to it.
Also her letters from home, one was from Mark, one was from her family, about the kids, so I'd assume they were with family.
Oh, and when she said she was gutted that marc wasn't there to meet her, and then mentioned her kids, it was in reply to questioning from Ant and Dec about Marc, she said 'I'm gutted he's not here actually, but at least my kids are' and then looked to Ant and Dec for reassurance they were actually there. It wasn't an after thought, just an answer to a question that wasn't about her children.