All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There is no harm in accepting this reward even if it is offered by a Christian charity. The Prophet, sallallaahu ‘alayhi wa sallam, accepted the gifts offered to him by non-Muslims, such as Caesar and Al-Muqawqis. Al-Bukhari entitled a chapter in his Saheeh: “Accepting gifts from non-Muslims,” in which he cited several ahaadeeth in this regard.
However, you should be cautious and mindful of such institutions as some of them may offer gifts for the purpose of luring Muslims into giving up their religion. It is most likely that such institutions do not give except to serve a hidden purpose.
Allaah knows best.