You need a separate office space (or else a room with a door, where you work and they don't go). Treat it like the kids are at daycare: all bottles, meals, etc. made beforehand or the responsibility of the nanny. Set defined break times when you will see the kids, and otherwise they're not to bother you unless there's an emergency. Trust the nanny to deal with their difficulties, that's her job. And encourage her to take them outside, it's nice out.
Good luck!