Why wouldn’t the secularization of Halloween be analogous to passeling an avodas zorah which Gentiles can do? I therefore think that the issue avodas zorah does not apply to the modern observance of Halloween. However, there is still an issue of chukas haGoyim.
Although trick or treating is rare in our communities I think that one should give treats to any kids who do show up mipnei darchei hashalom. It goes, I think, without saying that Jewish children should not participate.
P.S. To all of you want to take me to task for “paskening without a license”, please note the qualifiers, “I think” and “why wouldn’t”.