upgrading a child ticket

I have booked a trip to JFK for next year, 2 adults and a child. I am looking to upgrade to PE with miles. I booked the fare on VA website with a hotel as well. I spoke to VA to check my fare was up-gradable and they confirmed it was and that before the tickets were issued I needed to get my child's ticket issued as an adult ticket to process the upgrade. I had not experienced that before but didn't think any more about it. I now have the miles for an upgrade so I have just requested the tickets only to be told that for the child's ticket to be ticketed as an adult I need to pay additional taxes £78 and that is before I even upgrade. Has anyone come across this before? In all my calls to VA no additional charge was ever discussed and I was lead to believe that requesting my child's ticket as an adult was just a formality for an internal computer system requirement. Any advice welcome.