we had this misses forgot to post the letter took about 6 months to get back the payments ours was back dated, later they also wanted my 2007 accounts statement which took a while to get, I argued that what has that got to do with child benefit surely children born here working here with an irish parent get the benefit anyway, they had 2006 -2008 -2009 -2010 (which they had i didn't supply any of them) so why do you need 2007, My only thought can be that they have done a review on everyone and so when/if it become means tested they can implement it straight away.
on the up side we have not had a letter in a year from them, It was a big pain, but you can see the need to do it..