Welcome and congrats on the new vehicle! Unfortunately, you cannot play one audio source to the front speakers and another to the rear at the same time. With the headphones, you can turn off the rear speakers (as a courtesy to the rear seat passengers) and play another audio source to the front or you can leave the rear speakers on, but they will play whatever audio source is playing from the head unit.
I'm not sure about the external speakers since I've never tried that. Someone else on the forum may have some experience here. Before you know it, your kids won't even watch the screens in the car because they'll be playing on their phones or tablets.
2014 Q50 Hybrid Sport
2016 QX80 Limited