When we first allowed our kids to have charging privileges, I asked about putting a limit on the amount. IIRC, they suggested that we check on their spending occasionally (long before there was an app) and if it approached the limit we had in mind, we could go to GS and have charging privileges removed. We'd usually get a printout every 2-3 days anyway so it wasn't difficult but wasn't convenient either.
They were good though. I was expecting them to go crazy in the shops but I think they bought some popcorn before the shows.