A parenting influencer is opening up about what she’s calling the "worst day of her life" -- revealing she accidentally ran over her toddler in a terrifying freak accident.

Kelly Hopton-Jones shared the ordeal in a lengthy IG post ... saying she was heading out Wednesday with her 4-year-old daughter Lily to grab donuts, while her 23-month-old son Henry stayed home with her husband -- but in a split second, he darted out of the garage and into the path of her car.

She says they rushed Henry to the ER immediately -- and incredibly, X-rays showed his chest, legs, and neck were all normal, while a neurological exam found no signs of head trauma.

Henry didn’t walk away completely unscathed, though -- he suffered pelvic fractures and a few abrasions that’ll take time to heal.

Still, Kelly says it feels like a miracle he survived -- adding she and her husband have been replaying the moment over and over, trying to make sense of it ... but ultimately acknowledging it was a tragic accident.