Again, it depends on which way you are willing to look at it.
I could argue that a family with 6 kids had 5 more people that are willing and able to save the earth than a family with 1 kid.
And what if one of those 6 kids goes on to research/invent/contribute to something world-changing and miraculous for the environment or society?
What if the only child goes to become the CEO of Shell and chooses to pollute hundreds of lakes and rivers and poisons thousands of animals in the pursuit of oil and greed?
What if that one kid is in disposables for 4 years, and the 6 are EC'd 100%?
What if the 1 kid had a 3000sq foot house and his parents use disposable EVERYTHING and own 4 hummers? What if the family with 6 kids lives in a tiny "green" house and uses public transportation and walks everywhere?
What if the one becomes a narcistic sociopath and rapes and murders rapes 36 women? What if one of the 6 opens and runs a sanctuary for battered women and fundraises millions into rehab and family councilling programs?
We could "what if" all day, but the fact of the matter is, you cant prove AT ALL that 1 child is more responsible or has less of an impact on the earth/environmnent/society than 6,
nor can you prove that 6 children are more responsible or has less of an impact on the earth/environmnent/society than one. Because that is assuming.
There are infinite possibilities lying in each and every one of us.
But...what about when those children go out into the world...do 5 adults living lightly thread more lightly in total than one adult with an average footprint? I don't know.
And what about when those 5 people have 2 or more children each? The impact surely is greater.