TBH we don't know for a fact that they don't already do rent out the Castle/Apartment to people who can afford it.
If you're a billionaire and $50k is small change, and you know it'd make your childs biggest dream come true, why wouldn't you phone them up and just enquire?
The best way they could make the castle chargeable would be to say add on extra $1k/$5k/$10k whatever a night to the price, and give that extra to Make a wish and other charities.
I don't see why it's unfair that rich people spend their money how they want, most wealthy people I know, and most super rich people who you hear about in the press do realise how fortunate they are and are more than happy to be generous to those who need it.