I've taken my kids on some pretty busy streets (10 and 12) but it's not their riding sense that makes me edgy, it's the lower level of bike handling skills... we were riding down a street one day and my son skidded his rear tire on a 40 mph street, so we got to train on what to do when that happens. it can be nerve wracking though.

I like him right in front of me and we chatter back and forth about what to do with traffic, lights, stop signs etc. There's no substitute for experience though, so i'd definitely encourage you to ride on roads with cars if it's not a dangerous road.