I’m sorry, but I don’t understand your question. It is not possible to add children to the half-fare card.
Are you referring to the Junior Travelcard? (Information about this here)
If you have bought a Junior Travelcard for your child, the card will be visible to train staff and can be scanned from your smartphone.
If you don’t mean the Junior card, please specify your question. Further information for children can be found here.
I wish you a nice day.