Teaching is Important
Perhaps we're fortunate to live in a rural area, and the only high speed traffic is on the one through highway and rural blacktops. Having a 10 year old daughter we ride our local rail trail whenever practical, as it provides the best safety. Unfortunately, it is a very slim minority of rural kids that have a trail within usable distance.
Our sidewalks are not in well enough repair to ride bikes on, therefore she has always ridden on low volume city streets. We have worked very hard teaching her proper riding etiquette, as it only takes one accident.
I didn't realize how well she was doing until some of her classmates joined us for a ride one day. I was sure one of them was going to have a car accident before we could get home.
Would not dream of taking her on the highway with broad shoulders that many local road bikes enjoy. However we have taken her on the medium speed rural blacktops when the trail was not available. We kept her very well guarded with one adult in front and back, and communicating the occasional vehicle position.
Trying to walk that fine line of keeping her safe, and preparing her to handle riding on her own someday. Ask in 10 years, I may (and may not) know how we did.