three into two won't go

The Guardian has an interesting report on a call from some doctors for people to be encouraged to stop having so many children.
It seems two is the magic number otherwise we'll have too many mouths, too many carbon footprinting people and, as Malthus reckoned we'll consume ourselves out of existence.
As I'm expecting my third child in a couple of weeks and like to think I'm pretty ecologically-minded I find the adoption by some environmentalists of population control ideas interesting.
Certainly, this is an issue which can really rile people. You can slag people off for flying short haul every other weekend and they'll study their fingertips and shrug or laugh. But telling someone that by having a third child they've make a catastrophic choice for the planet and you'd better duck quick.
So firstly, bring on the debate - but let's be clear about what it entails.
The authors of this call in the British Medical Journal are part of the Optimum Population Trust which says it is "the leading  think tank in the UK concerned with the impact of population growth on the environment". It's a small field so leading is not that difficult.
It's blog has links to different articles, many on immigration, and all seems fair and reasoned.
My problem is that population control is all about someone else choosing who gets to breed - and quickly that becomes a question of what kind of people get to have children. and what kind of children.
Most of the people in my village with three or more children are AB1 social class - they're exactly the type who will find a way round any moves we might make for population control. The same as they mangle rules on education to make make sure their ones get in to the best state schools.
I fear that we'll end up with the state telling poor people to stop breeding.
Consumption needs to be curtailed but I don't see fewer children as the silver bullet because it presume they only consume and don't contribute.