I do not have custody of my children, but do have visitation rights. Am I still entitled to use the Hague Convention to get my children back if my 'ex' has taken them abroad without my permission, and aims to settle there?

Yes, in principle, providing both the countries involved are bound by the Convention and you were exercising some kind of parental authority (in Hague language misleadingly called 'rights of custody') according to the laws of your home country when the children were removed.
Moving Abroad