Olympic gold medalist Shawn Johnson just revealed her children's school was placed on lockdown as the Nashville shooting went down nearby ... and while her two kids are safe and back home, she's devastated for the families who can't say the same.
The former Team USA star shared the terrifying update on Monday ... saying, "I haven't been able to catch my breath since reading the news and getting a call from our school that they were on lockdown as well given the news. Shaking. Crying. Heartbroken. Horrific."
"Our babies are now home with us. I feel lucky and blessed but still so incredibly sad. No parent or family should ever have to deal with this."
Johnson -- who is married to former NFL player Andrew East -- detailed her chaotic experience ... saying she witnessed ambulances, police cruisers and buses full of kids being taken to safety, as well as frantic parents searching for their loved ones.
"Today has changed me," she added.
Johnson and East have two children together -- 3-year-old Drew and 1-year-old Jett.
At least three children and three adults were killed at The Covenant School in Nashville ... as well as the alleged shooter, who is being described as a 28-year-old woman.
"Praying for the families whose baby won't be coming home from school today and the families who have lost a piece of themselves," Johnson added.
"I also pray for the children and adults who may have witnessed, first hand, the heinous acts of violence inflicted today."