have you looked at the Newfoundland? these match your criteria almost exactly. we have never heard of one untill we started looking for a dog. we did all of those "find your breed" sites and these kept coming up on top. so we gave them a try. absolutely wonderfull dogs. great with kids, dogs, anything really. extremely laid back dogs untill needed. they were water rescue dogs so they were bred to be out of the way untill needed.
here is a pic of Fezzik and our two kids, Ryan and Alex. Ryan is the older one. as you can see they are picking and prodding Fezzik just like all kids do. when Fezzik has had enough he will start by licking one of the kids. if this does not work, than he will get up and move. have never heard him bark or growl at any kids.
he will bark at new people, but if they stay for more than 5 minutes than he guesses they are gonna stay and will go back and lay down.
G~