I have a canon rebel T6 what lense, external flash should i get to take pictures of kids?

I’m new to having a camera and I’m not sure how everything works with my dslr. i have a T6, and i would like to take pictures with flash without the kids squinting and that it is fast when I shoot. Also do you know of any websites to learn about aperture mm, and all of those details? I am not sure how i should be choosing the lenses. Thank you!