It probably just depends on how full the flight is. If the plane is empty, there will be discounts for families, because they want to sell tickets, and families are price concious. Solo travellers usually are not.

If the plane is on a popular route, you'll pay through the nose, and there sure as hell won't be any group discounts. You'll pay almost full price for a two year old, because they know they'll fill the plane no matter what.