Surely the answer is for the club to allow as many upgrades as you like - provided the seat is upgraded to the price of the full seat cost (minus the season ticket cost already paid). This would mean that the club couldn't be exploited by people using a child seat for adults with upgrades below...
Not sure if this has been mentioned elsewhere but I'm disappointed with the clubs new stance on upgrading seats.
I have three season tickets in the East Upper as I've always taken my daughter and son along. At the start of this season my son got a Saturday job which meant he couldn't always get...