HW, pricing is not cost-plus. If it was you would have a single price for all seats regardless of when the ticket is purchased.
The correct price, from the airline's persoective, is the maximum price the punter will pay on that day. Anything more and you don't get the business. Anything less you lose opportunity.
That lighter kid could be very well offset by the fatter parent. In a perfect world we would pay per kg including everything taken on the aircraft. But impossible to implement.