All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If the mentioned Christian association offers you to design a web page which preaches Christianity, spreads its ideas and beliefs, defames Islam or creates doubts about it, etc., then you are not permitted to cooperate with them in designing that page. Since this work is a direct involvement to strengthening their Kufr and evils, cooperating in this work is a major Kufr (disbelief) that annuls one's belief in Islam and puts him out of the Muslim Nation.
Your need of money does not allow you to commit Kufr. May Allah protect all of us from such sins.
We ask Allah to satisfy you by His Grace and grant you lawful means of sustenance.
Allah knows best.