Try to get hold of this Indian book: Vedic Mathematics or Sixteen Simple Mathematical Formulae from the Vedas (https://www.amazon.com/gp/product/8120801636/).
I have also heard of a Trachtenberg Speed system (https://www.amazon.com/Trachtenberg-Speed-System-Basic-Mathe...).
For children? Nine times table on your fingers is good.
https://youtu.be/xBTGKiVgWcA
Try to get hold of this Indian book: Vedic Mathematics or Sixteen Simple Mathematical Formulae from the Vedas (https://www.amazon.com/gp/product/8120801636/).
I have also heard of a Trachtenberg Speed system (https://www.amazon.com/Trachtenberg-Speed-System-Basic-Mathe...).