HMRC should explain in writing why your Child Benefit claim was refused and the date the decision was made. Check the reason carefully, as it may relate to responsibility for the child, residence, or another eligibility condition. If you disagree, you can usually ask HMRC for a mandatory reconsideration within one month of the decision, then appeal to an independent tribunal if the decision is not changed. Check the latest guidance on GOV.UK, and consider getting advice from a benefits adviser if your circumstances are complicated.