So apparently Unicef UK know better than the authors of the United Nations Convention on the Rights of the Child 
UN version:
Article 2
1. States Parties shall respect and ensure the rights set forth in the present Convention to each child within their jurisdiction without discrimination of any kind, irrespective of the child’s or his or her parent’s or legal guardian’s race, colour, sex, language, religion, political or other opinion, national, ethnic or social origin, property, disability, birth or other status.
Unicef version:
Article 2 (non-discrimination) The Convention applies to every child without discrimination, whatever their ethnicity, gender, religion, language, abilities or any other status, whatever they think or say, whatever their family background.
(I saw it on twitter, but I forgot to copy the link to the tweet calling on Unicef to correct it. Both docs available at www.unicef.org.uk/what-we-do/un-convention-child-rights/)
Google does suggest historic links/collaborations between Unicef and Stonewall, to nobody's surprise. This change is deliberate, and must have been made by someone with a vested interest in promoting gender ideology - what other reason to change a clear biological definition to an airy-fairy feeling? This change has no benefit to children (and potentially harms), which seems grimly ironic given the purpose of the whole document.