The Netherlands is amongst the countries that has withdrawn humanitarian aid to Gaza through the UNRWA because of an accusation that 12 out of 13000 employees of UNRWA somehow helped Hamas. The claim is based on "confessions" obtained by the Israelis through torture.
The most efficient way to send donations is probably by direct bank transfer. Details of accounts can be found here:
https://www.unrwa.org/donate/questions-about-donations