Attending a party in a church building

Absolutely not.

Generally speaking, one may not enter even a courtyard of an avoda zora or even traverse a road leading to avoda zora (when there's another way). Many even have the custom to cross the street when passing an open church, and such was the Rebbe's practice. In fact, the Ritva (Avoda Zora 11b) and the Rashba (Tur Y.D. 149) say that one must give up his life rather than enter a church courtyard.

There may be specific exceptions (and even those are in dispute and a rav must always be consulted for a heter) in cases of great need and in a manner where bystanders cannot possibly suspect that one is going to the avoda zora (such as a repairman clearly dressed as such carrying tools etc. or a polling station in a church, where everyone is going to vote), but none apply in such a case. A birthday party is certainly not in the category of a great need.

And even in a case where technically permitted, we are admonished by Chazal: "Distance yourself from unseemliness and from things similar to it" (Chulin 44b). Furthermore, entering such a place (without a legitimate heter) causes spiritual impurity to adhere to the person ר"ל.

Additionally, please note that attending any non-Jewish/non-kosher party, regardless of location, is problematic and should also only be done with qualified rabbinic guidance.