I agree with this idea entirely. I travel with two kids that barely weigh 90 pounds put together. I should pay for their seats. But why am I paying the same price as two people that total 450 pounds?
Price tiers every 50 pounds.
Problem with your argument is that your children could be replaced on the flight by other passengers that would earn them more money. From a profitability standpoint, deducting anything for "underweight passengers" is going to cost more money, and that simply won't happen.
I think the only reasonable solution is to charge for those that are more wide than the seat allows. Make special seats available, at a reasonable increase in rate, but require that a passenger fits in his allotted seat. If they do not, they must pay for the space of two normal seats or for the upgraded seat if one is available.
Personally, at 6-3, I find standard seats to generally be far too short. I prefer emergency exit seats with a bit more leg room. I honestly wish companies like southwest would do a better job of holding those seats for longer passengers, and that other companies would require people in those positions to be something other than 5-5 130lbs.
As it is, flights are designed for the average sized person. Tall or wide is generally unaccounted for. Punishing someone for being tall is a bit strange. However charging the fat guy for the extra space he occupies is reasonable given that being fat is something that one can control.