Bart's body moved on his own as he saw the green spell coming to Garth. But as he tried to just move with him out of the spell way, he realised they were surrounded by it. Bart ran as fast as he could around Garth to protect him from the blast, making a human shield.
The second he stopped, Bart's vision turned black and he felt himself hit the floor. You felt your heart sink as you watched Bart's figure fall on the ground.
With their new found anger and the villain's exhaustion after the last attack, they managed to knock him out. (And maybe they hit him with a little more violence than necessary)
"The spell he was hit with de-aged him fourteen years, is simple enough that I could-" Zatanna's diagnosis was cut short by Jaime's voice.
"Fourteen? But Bart is fifteen! And hee looks older than one!"
"That's because of his metabolism, it makes him age faster than normal" Barry explained.
"Uh, In that case it would be safer to age him a couple years at a time and let him rest for a day between them. Meta-humans tend to react weirdly to magic and his powers are very intense"
Once Zatanna left, it was mainly the team left. All ayes settled on a very scared, young, eight year old Bart who was running out of the bed (at normal speed, weird) and settling on the ground in a defensive posture.
So he was eight now.
He also had one of those disgusting collars on, Robin deactivated it in seconds.
His clothes were different though, he was hearing a white t-shirt with some numbers on it and white trousers. You wanted to punch the reach so fucking hard.
The de-aged Bart realized that he couldn't ran away from then and started analysing them. Then his eyes stopped at a glass of water and he smashed it in the floor. Everyone was very confused untill they watched the kid grab the biggest piece and hold it like a knife. His hand was bleeding-
"Get the fuck away from me!" Terrified. He was just trying to survive. It was weird to hear such a small kid swear like that, let alone a kid like Bart.