This is an irrational fear. In 2016, there were about 245 children under 14 killed by cars as pedestrians in the entire year: https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/.... There are about 75 million children under 18, so let's say 40 million between ages 5 and 14. That's 0.6 child pedestrian deaths per 100,000 ambulatory children. That's about 1/5 as likely as the risk of a white person being killed in a homicide in any given year.
Moreover, about 332 children ages 5-14 were the victims of homicide in 2017: https://www.cdc.gov/injury/wisqars/LeadingCauses.html. Us urbanists ridicule suburban parents for worrying that their kid will be shot if they live in an urban neighborhood. Your child being killed by a car while playing in the street is even less likely, and having any special anxiety about it--apart from the general terror of parenthood--is equally irrational.
We could certainly do better on this front. I'm pretty lucky to live in a pre-zoning code suburb with narrow little streets and houses close together, where I can let my seven year old play outside with my one year old as long as they're more or less within visual range. Neighborhoods like mine are illegal to build today. But, I grew up in a standard suburb, and we played on standard suburban streets starting at age 6+. It's pretty common outside big cities and pearl-clutching millennial parents.