Mr Boombastick
When travelling on any Concession ticket booked in Economy you will not be upgraded to any higher class of travel regardless of your status or children's age. The policy is NOT to upgrade staff, if the cabin is full Revenue Pax will be upgraded but never staff.. That is the policy.
Also they will not upgrade revenue Pax to accommodate Staff Pax on ID90 tickets.
Certain Staff groups can book tickets in the premium cabins, but they have there own rules, and you seem to know that as an FO one cannot book tickets for your children in J class until they turn 13.
These are the policy's, possibly on one very rare occasion an upgrade might have happened, but the system will catch up with the errant station and re-clarify the policy