Q. How do I reference a UN convention?

Cite them Right contains guidance on referencing United Nations treaties and conventions. To ensure you have the fullest information, we recommend using the United Nations Treaty Collection.

As an example, if you were referencing the Convention on the Rights of the Child, your reference would look like this:

Convention on the rights of the child (1989) Treaty no. 27531. United Nations Treaty Series, 1577, pp. 3-178. Available at: https://treaties.un.org/doc/Treaties/1990/09/19900902%2003-14%20AM/Ch_IV_11p.pdf (Accessed: 3 July 2020).

In text, your citation would be: Convention on the rights of the child (1989) or (Convention on the rights of the child, 1989) depending on the construction of your sentence. 

(For a bit of context on this particular source, please see the Unicef website, which includes another version of the Convention of the rights of the child. This English version starts on page 31 of the document included in the reference above.)