Technically your wife could be breaching the visa conditions:
Paragraph 6 of the Immigration Rules defines what benefits are consider as public funds for the purpose of any consideration of the Immigration Rules and it is this which determines whether they have breached their conditions by claiming them. In that list, Child benefits are classed as public funds.
However, there are exceptions when claiming benefits and your wife may fall within these exceptions:
"There are occasions when people who are subject to immigration control can claim benefits without it being classed as breaching their conditions. In these cases the fact that they have claimed public funds will not affect their
application for leave"
The exceptions list can be found here:
http://www.ukba.homeoffice.gov.uk/sitec ... iew=Binary
I will suggest you get legal advice to clarify whether your wife falls within these exceptions.
Best Wishes