Kelly Mochan had some of her six kids with her at the time as she rampaged through a school playground and smashed 53-year-old grandmother Tonia Vincent on the head.
Pupils and parents looked on astonished as teaching assistants at Pyle Junior School, near Bridgend, South Wales, pulled the women apart.
The court heard that Mochan took the baseball bat into the playground because she suspected Katie Vincent of having an affair with her husband.
But Katie, 31, had sent her own mum Tonia to collect her children - so Mochan, 31, clubbed the grandmother with the silver baseball bat instead.
Prosecutor Mike Hammett said: "She told the gran: "I'll get your f****** daughter" as she was going through the car park.
"There was an exchange of words and she said she was going to kill Katie for what she had done.
"Mrs Vincent reminded her there were children present and she took a baseball bat from her car, raised it and hit her on the head.
"There was a struggle, they both fell to the ground and the teaching assistants intervened.
"The grandmother was then taken to hospital with swelling and bleeding from her head."
Mrs Vincent said: "I thought I was going to die in the school playground in front of my own grandchildren.
"She was screaming and yelling, and her language was atrocious. She kept saying she was going to kill Katie.
"She walked over to her car and started getting something silver out of the boot. I didn't know what it was at first - but then I saw it was a baseball bat.
Her face was so angry it was distorted. Then she swung at me out of the blue and I felt it crack against my skull. I could feel warm blood trickling down my head.
"The doctors told me I had a very lucky escape. She was a lunatic. A mother of six should be ashamed of herself for behaving like that.
"I just hope she gets treatment for her anger - she needs it badly."
Mochan claimed she kept the bat in the car for her children to play with and had taken it out as a warning.
"You are a mother of a large number of children and a very good mother to all those children.
"This occurred at a time of domestic turmoil, which has now resolved itself and you apologised at an early stage and expressed remorse."
Mum-of-two Katie denied she had been having an affair and said: "It was all untrue nonsense.
"She had threatened me earlier in the local supermarket and the police were called. I've been too afraid to pick the kids up from school.
"I'm just so glad the summer holidays have started."