The athlete's son, Liam Mikael Kowal, was being pushed in a stroller through a crosswalk by his teen aunt Saturday afternoon in Hawthorne when they were struck by a 72-year-old woman, according to police.
The suspect, identified as Donna Marie Higgins, tried to flee the scene, but witnesses followed her and blocked the vehicle until authorities arrived.
She was arrested on suspicion of felony drunk driving and felony hit-and-run. She was booked at the Hawthorne jail in lieu of $100,000 bail.
Kowal posted the devastating news on Instagram.
"The support we have received from the Martial Arts world is unreal. From #BJJ to #KravMaga to MMA. Tears are running down my cheeks as I look at this picture of my little boy and read the message from @bashboxing and @cxf_mma," he wrote.
A GoFundMe account has been set up to help the family.