Both DH and I are Canadian and worked for a year in Canada before we came to the UK.
Actually, sorry, the Canadian thing doesn't work for Tax Credits. However it definitely is OK for you to claim Child Benefit, and you should do that as soon as possible, given that the claim can only be backdated a maximum of three months.
Use the link kindly provided by yankeegirl, on page 4 of 7, in the right-hand column, you will see that Canada is listed there.
Yankeegirl, that chart on page 6, all those ticks and crosses, that totally sums up just now complex this whole matter is.