(Reference for a preliminary ruling - Citizenship of the Union - Article 20 TFEU - Directive 2003/86/CE - Right to family reunification - Union citizens who are minor children living with their mothers, who are third country nationals, in the territory of the Member State of which the children are national - Permanent right of residence in that Member State of the mothers who have been granted sole custody of the Union citizens - Change in composition of the families following the mother's remarriage to third country nationals and the birth of children of those marriages who are also third