Honestly, I'd chase for the compensation of the lifetime earnings of an expected 'life'.
If those children had grown up, held an average-paying wage job until retirement age. I'd want that income. That's what a life is worth.
Problem is that those earnings are the children's. They aren't around to sue for it and if they were then there isn't much to sue over.
It's just the way it is. The fine isn't supposed to be punitive but reflective of the emotional distress of the parents and any monetary losses they have incurred. I believe the actions of Thomas Cook need to have been criminal for punitive damages to be considered.