How to make pictures lots of kids so everyones face not blurry?

I have Nikon D850 with portrait lens AF 105mm f/1.4. Im just learning how to make good pictures , we purchased this camera not for business , we have kids and want to make every possible memory. Im using A mode, but often when I make picture one kid face is in focus another one is blurry( I have twins so it rarely happens to get good picture with both kids in focus). What setting do I need to change? Kids birthday is this weekend and Im the one who will take pictures of all kids.

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!