A small triumph!
It turns out that Arriva had a link missing from their website - and it just had to be the child 4-weekly pass link. (Nice of me to flag it up for them, wasn't it?!) A kind lady got me the old link, which will remain active for the next month or so until they get the new website corrected.
HALLELUJA! Justice prevails, and I've got those two passes ordered. It's a pity about the hiatus caused by the Bank Holiday, but at least it's sorted out now.
I'm just too damn good at spotting inconsistencies in websites!