The best you can do is teach them about bike and traffic safety. I discussed in length, provided her reflective gear and bright lights, and rode behind my own daughter at age 8, on local rural roads including a stretch of a state highway (55mph speed limit). She handled it well and proved to me she could ride safely on the road, and now at age 10 rides these roads quite a bit, including solo every morning. In fair weather, she rides her bike to school (~6.5 miles, nearly 1/4 of which is on the highway I just mentioned)