Roads around my house are quit narrow, with no shoulder(country roads) usually with a 5' ditch right next to it.
I ride in fear when I go alone, let alone if my daughter was riding.
That said, some of the more remote roads with very little traffic my 10year old does well. She has a full Road Bike (Trek 1000) and can keep up a good clip.
I would say between 8-10 they should be good enough and know better to not ride out into traffic and to stay safe, especially with an adult riding behind.
We usually go as a family. Wife leads, Daughter in the middle and I bring up the rear. Also I stay a little farther out like said above.
Good Luck, good to see wer're getting them away from the TV!!!