I would like to know if it's possible to chose a child's sex during copulation.
If possible, please how can it be done

Yes. Look up Shettles method.

Have this in mind;
The female sperm cell is slower but lasts longer than the male sperm cell in a vagina.

The male sperm has less stamina / durability but is faster than the female sperm cell.

I trust you can do the math yourself.

It is. But from my understanding of nature, when we interfere with the natural order of things, there are consequences.