As long as you are in control of the situation I think this sounds fine as if they were walking next to each other not holding hands and (god forbid) one got hit, the other probably would too anyway, make sure (although you most likely already have) instilled and enforced the road safety rules to all of them
I think I might try the box idea - sounds like a good shock tactic! My DS is 6 - when he started school I was worried that he didn't understand how dangerous cars can be as they don't teach road safety at his school. I spoke to his teacher about the issue and she introduced the Beddy website to me (www.beddy.co.uk). It's an interactive website that gives parents and teachers resources to help educate kids about the green cross code and the importance of road safety. We use it all the time as it makes learning about road safety fun and interactive through games, lesson plans and worksheets. I've found that he has started to recognise things from the games such as lollypop ladies and pelican crossings when we walk to school which is great progress! I think they do school visits with their trucks too. Hope this can be of some help!