When it first came out, the Trick or Treat interaction would complete by using the bowl of candy that you can place outsider your house or by using the trick or Treat interaction on visiting sims. As far as I know, they're actually supposed to knock on your door to get the interaction, but kids just show up and stand around on the lot instead of knocking now.
The tradition does complete by exchanging gifts. I usually just put some fruit or baked goods in my inventory and give out "friendly gifts," to complete the tradition. Like drinking, and some other traditions, you need to do it three times for the tradition to fully complete.
