This is more than a retailing issue.
The conditions make it hard to prevent it being retailed inappropriately. It is valid with an adult travelling on a season ticket for example, so buying this ticket on its own is perfectly valid. Even if sold with an adult ticket, there is nothing to prevent the adult and child travelling separately.
The issue is that the ticket has conditions attached to it that are not documented as a restriction, and hence there is no way of enforcing that customers are informed of the conditions at the point of sale.
Easily solved, create a restriction code SEKIDS, document that it is only valid for off-peak travel when accompanied by a fare paying or season ticket holding adult, and well programmed journey planners and TVMs will display the restriction before selling. Won’t completely eradicate inappropriate selling, but at least the customer has a fighting chance of getting the right ticket.