When I read the OP I thought it was about treating a boy and a girl equally.
I am in absolute agreement about doing what the parents want, but I was just wondering how far this goes. Do they both get equal quantities of presnets on both birthdays.
However, there could be away round this, could you take both children out for a meal to celebrate the older girl's winning the competition? It would be clear that the treat was to celebrate the 10 year olds success, but he children would benefit equally.
It doesn't have to be a big deal, MacDonalds or Burger King, but you would be treating both children equally.
Like others I think it is a ridiculous rule and fraught with problems in both the long term and the short term.