Looking for a master SLR or mirrorless camera, which one can meet the needs of children during cloudy indoor capture sports?

First of all, you need to determine your needs in detail, whether you require home photography or commercial children's photography.

Children’s snapshots, sports. Whether it's cloudy or not is not a big problem, as the outdoor lighting is generally sufficient.

Capture requires you to have high-speed continuous shooting. Generally, high-speed continuous shooting cameras are flagship cameras of various manufacturers.

Shooting during sports requires a very advanced focusing system. For example, the Nikon D5 has 153 focus points and the focusing speed is extremely fast.

Or the SONY A7R2 can provide motion prediction. The focusing system ensures that you can take clear and sharp pictures when shooting children in motion.

On cloudy days, you can rely on a large aperture lens and post-production to deal with it, and the problem is generally not big.

So if it is for home use, choose a flagship model with APS-C format (APS-C continuous shooting is generally faster because the pixels are lower)

If it is for commercial use Yes, let’s go to Nikon D5 and Canon 1D X2.