"Domestic legislation provides that both Child Benefit and Child Tax Credit cannot normally be paid in respect of children resident abroad. However, under provisions in EU law on social security coordination within the European Economic Area (EEA), Child Benefit and Child Tax Credit may be payable to European migrants in the United Kingdom in respect of their dependent children resident in another Member State. The provisions relating to payment of “family benefits” for children resident in another Member State are in EC Regulation 883/2004, but they have a much longer pedigree. The rules apply to all European Economic Area countries, not just the United Kingdom."
As long as United Kingdom and other European Countries belong to the Union they must follow its own agreements to avoid discrimination of other nationals and avoid building new barriers between the countries.
Unfortunately it seems that someone in the UK forgot to follow the rules which have been agreed with the other countries.