Au pair is not just a word, nor is it a synonym for childminder/babysitter or cleaner.. An au pair is an official status that can only be allocated when you sign and register an official Belgian model agreement at the municipality, regardless of whether the au pair is a European citizen or not.
If you have not signed an official model agreement for au pairs, you are not an au pair, but a housekeeper and you are entitled to a minimum wage.