If you've not yet made a purchase, they'll be within a drop-down menu on the subsequent page once the desired number of adult tickets has been selected.
In all other instances, children's tickets can be gotten from within customer accounts on our website. Log in there and head to 'My Tickets'. Click on the 'Details' button on your order, then click 'Add Add-on'.
If you've forgotten your password, it can be reset from here: