i think in ticketek under your barcode add a 1 to "upgade child 1-5 $0"
at least that's what i did when entering my booking in previous weeks - 1 barcode 2 tickets $0 cost
that's before mfc started to directly download reserved seat member's ticket to your phone's mfc app. i think in your situation as a reserved seat member you should ring the club. alternatively i hear club is having the kiosk outside the ground and they maybe able to help, but that's leaving it quite late