Well, the show is a soap opera for kids...
The reason she transforms is because she was trying to make the little girl drink the "potion", but a janitor woman comes in and exchanges the drinks.
You can see the whole messing up in the final minutes of:
The reason of why she was trying to make the little girl drink that is -apparently- that for some reason she hates her and all those kids so she asks a witch to give her the potion. Why she hates those kids? No idea, but hey, who cares? It's just a soap opera... for kids.
