According to government figures the money they receive is taxed at source (taxed before the get it) therefore the amount issued is actually higher than the stated amount. I assume that if they save any it will be taxed in the same way as anybody else.
What I object to is the fact that certain people who have children living elsewhere in the world can claim UK child allowance for them. I know (because I work with some of them) that a percentage of their money is send 'Home' to their family's elsewhere in the world.
I have no objection to looking after your family, but if the taxpayer is paying for an extended family- That is wrong!!