I heard it rom an inside source that although Kerry hasn't the slightest idea how to raise her kids, she does care about them a lot, she just hasn't the faintest idea what to do with them. The nanny is a family friend who's been around since before Holley and max and she may be reluctant to leave them as she's the only stable influence the kids have. Apparently the younger of Brian's girls is a nervous wreck, and as a nanny myself I'd have never willingly left a job if I thought it would damage the child.
Also, my mother was a radio journalist back when Kerry and Brianna broke up and she interviewed him. He compared the children to puppies and admitted it was a mistake to have them. The listeners gave him a right bollocking and he fled to oz not long after.