I was surprised that buff orps and barred rocks weren't on your list - both have good disposition. I also have rir, who i like, but i wouldn't necessarily consider them the most friendly or docile - especially compared to my bo and br's. If it's between rir and nhr, i would go with nhr, which i have read are more docile - and which i'm getting ready to add to my flock. They're also supposed to be great egg layers. All of mine follow me all over the yard, looking for food. A couple of them jump on my shoulder whenever they have the chance.
I would not go with leghorn if you want your children to enjoy having a pet. Leghorns, who i'm just becoming acquainted with a little, seem very flighty to me.