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Number of Siblings, Religion, and Educational Attainment
Other things equal, one would expect the negative effect of large families to be experienced most forcibly in those groups that provided the least non familial support for high fertility, believing that each couple should produce only the number of children it can support. We would thus expect that non-Catholics (as compared with Catholics) would experience the most negative educational effect from large sibsizes.
By contrast, given the strong Catholic church commitment to large families (Blake 1966, 1984), one might expect that coming from a large Catholic family would not be as inimical to educational attainment as coming from a large non-Catholic one. Indeed, although in general, parochial schools have placed a great financial burden on Catholics, it is also true that such schools have provided a more supportive environment for the children of prolific parents than attention solely to tuition charges might suggest. As Greeley and Rossi have pointed out (1966, 273), parochial schools at times charged on a per family rather than a per child basis. Such schools are also known to charge less for each succeeding child in large families and to be generous with scholarships for those in need. In addition, families in Catholic parishes "network" with regard to children's clothing and other child-oriented resources. During the Depression, parochial schools often accepted the children of the unemployed without charge (Sanders 1977, 184-185). The parish frequently pitched in and helped the schools with whatever resources were at their disposal...."
http://publishing.cdlib.org/ucpressebooks/view?docId=ft6489p0rr&chunk.id=d0e5664&toc.depth=1&toc.id=d0e5664&brand=eschol
http://www.academia.edu/802918/Revisiting_the_birth_order-creativity_connection_the_role_of_sibling_constellation
http://www.familyfacts.org/briefs/35/family-structure-and-childrens-education
It can be taken from the text above that, in years gone by, particular groups that held that large families were a good thing made great 'communal' efforts to accommodate them, it seems that 'we' as a country are doing our bit too.
I did a fruitless internet search to find statistics relating to achievement and family size, the operative word is 'sibsize', in the hope of discovering whether large families were good or bad for the individual children within them. There has been some work done on this, arriving at vastly different conclusions.
The issue (quite literally) being children, and not financial burden, it appears that there are a great many factors relating to family make-up that have been studied at length;divorce/single parent, being a popular area for study.
It is, after all, the children that matter most here. Yes Family Allowance could be tailored to encourage us all to have a 'reasonable' size of family, but curtailing FA is bound to lead to a greater number of children suffering. [yea! Well done the Conservatives.]
Perhaps rather by-the-way, but the Windsors
are one large family group. Can it be they that encourage such large families further down the social scale?