What do you consider to be the mother tongue of your multilingual child? Would you say it’s literally the ‘tongue’ of the mother (even if that is the minority language) or all of the languages spoken by your child? The language where they were born? Or even the school language??
To be honest- who cares? What difference does it make to our children? Well, often schools and states do care… children have to indicate what their first language is for the school records, and many health care providers etc. ask for the mother tongue too. There are other form- filling situations where you are only allowed to put in one language… wouldn’t it be nice if we could just write “bilingual”, and leave it at that!