Child Benefits and Immigration

I am not sure why we are paying parents more than £712 p.a. per extra child when they have two already.  When the population is shrinking, encouraging the birthrate makes sense but when the population is expanding as fast as it is in the UK and we have pressures on schools, housing, roads, trains and the NHS, to name but five, why are we doing this to ourselves?
Most of UK population growth is due to children born here rather than net migration from within or beyond the EU.  Yet we are besotted with the migration issue which might even determine the outcome of the EU referendum.  What would be the ideal population size for the UK?  And how should we achieve that?
It is not as though destiny forces these extra children on parents who therefore need our help.  They have a choice in the matter and there is no shortage of products to assist them.  They decide to have extra children to please themselves, no matter how much they burden, and even annoy, the rest of us.  Any grandparent knows that, whilst one’s own grandchildren are wonderful, everyone else’s are dreadful and the world would be a better place without them.
Limiting the number of children per family receiving state benefit is not a new idea.  When the French did it, it proved too successful and they had to revert. The UK growth is too fast for that to be a problem here.
No doubt the loony left will tell the rest of us that large families have a “right” to child benefits for all, that it is a fair and necessary transfer from wealthy families, typically with few children, to the poor.  But children typically cost more than £13.70 a week to rear so actually large families make themselves poorer, especially when there is only one working parent.
Part of the reason for large families is cultural and imported with the parents or grandparents.  For most people in the world, notably India and Pakistan, children are their pensions.  But we have adequate pension provisions in this country and there is no need also to pay for these extra children.  And there are plenty of reasons why the Chancellor should stop doing so.