I think the difference is that a 3 year old barely-walking child tends to wander behind moving cars far less often than an 8 year old playing football.

1-4 year olds are the age group most likely to be injured in this type of incident.

https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5406a2.htm

I suspect older children are more likely to be able to be aware of their surroundings and have better gross motor skills to react.

That could be true but the 8 year old gets out of the way. I can remember two incidents on the news where a toddler was killed in a driveway. Tragic.