I didn't realise it was exclusively for travel within the area covered by the season ticket.That is a bookable option because the (adult) Gold Card holder can book a ticket for a child to accompany them when travelling on the route covered by their Gold Card, i.e. when they don't need to book a ticket for themselves. Normally it is not permitted for a a ticket with the CHAGC (child gold card) discount to be sold separately. Maybe you will get away with it though.
The terms themselves imply you can buy a ticket where your gold card doesn't cover the journey being made.
7) If the journey You are making is covered by Your Annual Season Ticket You do not need to buy yourself a ticket. However, if You are purchasing discounted Off-Peak Day Travelcards for accompanying adults or children and You intend to travel in a London Fare Zone not covered by Your Annual Season Ticket, You must also purchase a discounted Off-Peak Day Travelcard Zones 1-6 for yourself
If the holder happens to be a child (and we ignore the fact they don't have a Gold Card even if the paper it's printed on says it's one), I don't see why they can't buy a Gold Card-discounted child ticket in the same way an adult would be able to with a journey outside their Gold Card.