I don't know the price of the tickets, but if the kids go 5 times, and it's usually £15 a game, then why wouldn't the adults buy their kids season tickets for the £55 and shirt, instead of paying £75? Long term, the kids will end up supporting City and putting lots of money into the club. That's the whole point of the kids prices, to grow future fans.
Also if people wanted to go regularly in the South Stand, then buy a season ticket? It's really not a major deal, as @cidercity1987 mentions, we don't sell out each game so it's not like people can't see City.