Are there any free resources that explain in plain English all Maths terminologies, notations and symbols that kids and teenagers will be able to understand?
Notations, symbols, terminologies and words are cheap in math. Most math text/books actually have a page or two explaining what each symbol stands for in THAT particular book. What counts in math are IDEAS. That said, a good entry point into the world of mathematics is Book of Proof by Richard Hammack [0]. It's free.