If the earth needs us to have fewer children and the economy requires that we sustain the population, is there middle ground somewhere?

It's not that the earth can't support a lot of people, its that greed wont allow it.  The same is true for the economy.  

The economy doesn't require a level population.
It doesn't care if you have 100 people each buying $1 worth of goods ... or 1 person buying $100 worth of goods.
Either way $100 worth of goods was sold.

I remember a book I read years ago where people were required to die at by age 63.  That would fix the problem of caring for the elderly.

Fewer kids means more money.  

Ask China.  

Soylent Green

It seems the two are diametrically opposed. The economies upon which our society functions are based on consumption and consumers, whose growing numbers are placing increasing demands on the earth and its finite resources.  

I don’t think there is a pretty answer here. I also can’t foresee a timely solution being reached. We are befouling and depleting the earth at an unsustainable pace, and there seems no impetus among the powers-that-be for an immediate change of course. I recently read a million species are facing probable extinction in the next decades. We can’t feed all of humanity as it is now. We can’t break free of fossil fuels. Ancient freshwater aquifers are drying up. Arable land is disappearing. On and on.  

Then there is the culture of human idiots who still deny that man is having a destructive effect on the climate. 
To me, the ignorance and apathy are the worst of our avoidable problems. 

Among the solutions I can conjure are to refashion our societies to be more cooperative with the environment. We have to use the earth more wisely and efficiently. 

Exercise controls on human populations. I dare not delve any further here, lest I inspire the wrath of the parties of God. But we have to face the limits of our sustainability. 
 
Incentivize environmental initiatives and related commerce of every stripe, at every level of government.
Boycott every and all sources of commercially induced environmental destruction.
Vote out of office anyone who cannot get with the program. 

rnaybe