I'm pretty sure that once the fall festival has officially started, kids can trick or treat. Spooky Day is the final day to trick or treat. So for example, if your fall is 7 days long, they can start trick or treating on the second day of fall through Spooky Day.
My season is 28 days long and on the last Thursday of the season I got a notice saying Spookey Day is just a few days away and starting tomorrow kids and teens will be able to visit their neighbors and trick-or-treat.
Which meant Friday was the first day for Trick-or-treating.
The next day (Saturday) was Spookey Day and the final day for trick-or-treating and three days left until Winter which means in total, out of a whole 28 day season there are two days for trick-or-treating.
P.S.
I just discovered that the NRaas Home Opener mod interferes with trick-or-treating. The trick-or-treat option wouldn't show up until I clicked on the house and did NRass/Master Controller/Close.
Then the option showed up on the door. I selected it but me Sim never went up to the door and never got any candy and I haven't been able to get the option to trick or treat to show up on any other houses even after closing them in Master Controller. The trip wasn't a total lose though as my alien teen did find several space rocks lying on peoples lawns and they are a bit like candy for aliens since they can eat them to recover brain power.
But I can't help feeling a bit like Charlie Brown. "I got a rock."