Here's what UKBA state in their 'Guidance – public funds – v8.0'

..A person subject to immigration control is not considered as accessing public funds if it is their partner who is receiving the funds they are entitled to.
Child and working tax credits are claimed jointly by couples. If only one member of a couple is subject to immigration control, then for tax credits purposes, neither are treated as being subject to immigration control.
Hope that allays any concerns