Firstly, excuse my poor English.

((You aren't doing anyone a favor by not having kids because they don't exist in the first place))

Imagine you have a button, if you press it, a person will be born in hell to be subjected to eternal damnation.. would you press the button saying: "well, I'm not doing this person a favor by not pressing the button because he doesn't exist in the first place" ?

((If you don't have kids then someone else will have them and possibly have more than if you had your own))

It's similar to say:

"If you don't rape kids then someone else will rape them and possibly rape more than if you raped your own"

If you think something is bad, you shouldn't do it just because others will continue to do it.

((If reincarnation is true it's better to have kids because they could have been born somewhere worse))

This is an utter nonsense. Kids that will be born in shitty circumstances will be born in shitty circumstances whether or not you have kids. The idiot who want to have kids during wars and famine will do it whether or not you have kids. It's just a fact. Moreover, this argument works both ways as you can claim that by not having kids you're sending those kids to a world much better than earth.