How to make curved lines of a parent DIV from top & bottom?

I need to create a styled grid like this one in the image.

I think the image can make you receive the idea, but I need the borders more smoothly.

It may be a big div; has many small divs, but when we add a child(div) to the parent(div) it gonna take the curved border lines shape of the parent and its convex to the inside shape.

I need to make the view looks to the inside of the screen.

I hope that you got my idea. Thanks.