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. If you use a different referencing style, see our Referencing page for full guidance.
(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.)