Yes, Chang An West Road has a bunch of smaller shops selling Halloween costumes and masks and other holiday gear. The stores also sell Christmas stuff and general knick-knacks–maybe this stuff is mostly for export, but winds up in these shops.
I don’t know if it is worth it to make a trip all the way over there, but if you’re in the neighborhood, it might be worth stopping in to take a look. One word of advice: avoid the weekend, esepcially this weekend–the last before Halloween. It can get a bit crowded in some of the stores at this time.
RT Mart also has a bunch of children’s costumes, ranging from a pumpkin outfit for the younger kids to a princess outfit and superhero costume for the older ones. They also have plastic pumpkins and pictures of witches and other decorations. So, that might be a more convenient option.
Might Costco have some Halloween costumes? Carefour? I did see pumpkins in Wellcome, so maybe stores are getting into the Halloween spirit (and selling more things, too, of course). I think Wellcome is also selling those bags of bite-sized Halloween candy that is given out to trick-or-treaters.
Best of luck and Happy Halloween.