I don't know if it is stipulated in the companies articles of association Alice, maybe you could write to them and find out. I would imagine it would come into discussion if and when Madeleine is found.
All the relevant documentation is available from Companies House.
The section relevant to our discussion reads:
Objects2B. The objects of the Foundation are:
2B.1.1 To secure the safe return to her family of Madeleine McCann who was abducted in Praia da Luz, Portugal on Thursday 3rd May 2007; and
2B.1.2 To procure that Madeleine's abduction is thoroughly investigated and that her abductors, as well as those who played or play any part in assisting them, are identified and brought to Justice.
2B.2 If the above objects are fulfilled then the objects of the Foundation shall be to pursue such purposes in similar cases arising in the United Kingdom, Portugal or elsewhere.
Winding up2F. If any property remains after the Foundation has been wound up or dissolved and the debts and liabilities have been satisfied it may not be paid to or distributed among the members of the Foundation, but must be given to some other institution or institutions with similar objects. The institution or institutions to benefit shall be chosen by the Directors at or before the time of winding up or dissolution.
That seems perfectly reasonable to me but it does
not say surplus funds will be given to Missing Children.
The caveat of
if on 2B.2 and 2F should be noted.