The robbers targeted three vendors in 40 minutes on Thursday night.
The grandson was arrested by the LAPD on suspicion of murdering a man in Downtown L.A in a separate case.
Bomb-sniffing dogs were called in but nothing suspicious was located, police say.
“My heart is broken, I don’t know what to do without him,” said David Arbogast.
Before she was hit, she managed to toss her son out of the path of the vehicle, investigators say.