You can't mix and match different passengers in different cabins on the same booking, at least for ticketing purposes. On board then with the cabin crew's agreement then it's fine to do this. So you are on at least 2 PNRs here. The 15 year old can travel alone anyway, so that's not the issue. The 13 year old won't be travelling alone as it happens, the easiest way to handle this is to call up and ask for the tickets to be linked To Complete Party (TCP). This is the first part of the process some Gold passengers use to get someone sat next to them, but that first part is available to any traveller.