how to disable the fn function in keyboard?

My 2 year old daughter really loves playing with computers,
she enable the fn keyboard function, the "fn" key at the left bottom corner,
now one of my computer's keyboard doesn't work well, I have to use
the "fn"+ right side keys to type, such as "fn"+j would give me "j",
otherwise it would be "1",k would be "2", etc.

How could I disable that fn function?

Thank you very much!