If you already have an an Annual pass and wish to add another child, then it is possible to add them on a pro-rata basis, so that the pass ends at the same time for all children. At this point you can purchase a new pass to include all the children.
This means if you have 5 months left on your pass then you can add your extra child for 5 months. (upgrade includes £5 handling charge)
(must match the info stored in your app)