Because I'm not sure all 7 will have kids.
You need 2 children to replace a mum and dad. Then any more replaces the fact that a huge chunk of the population is now remaining childless. We are going to have an enormous elderly population in years to come. Who's going to look after them and support the economy? 
There's also the fact that a family with one child who live in an enormous energy-consuming house, don't recycle, constantly waste, eat high meat diets is going to be worse for the environment than a family of three children who are extremely environmentally savvy and you know, give back to the planet in their future careers too.