Thanks Shandy, I've just seen your reply.  That is my husband's argument, exactly, and the person he spoke to at Child Benefit confirmed.  We cannot get any answer from immigration about it, though, which would make me feel better.  My husband sent an email more than 21 days ago (their timeline for answering them) and has not heard back.  The entire thing is as clear as mud.
Seriously, don't worry about it, easy for me to say, I know, but you've broken no rules.
Child Benefit (CB) can be claimed by a 
person responsible for a child, regardless of 
whether the child lives with that person. The 
claimant cannot be a ‘person subject to 
immigration control’ (subject to exceptions – 
see footnote) and must be present and 
resident and have the right to reside here. 
The child must also be present and resident 
here. CB is classed as public funds. From 
January 2013 a child benefit high earner 
charge will be paid by a person or member 
of a couple earning over £50,000. 
So as long as the claimant (the UKC) is not subject to immigration control, then no rules are broken.