Trolltide is not exactly spooky, and this one is specific to Waterdeep. But there are kids in masks, tricks, and treats. From the Forgotten Realms wiki article:
On this day commemorating Waterdeep’s victory in the Second Trollwar, children run through the city acting like trolls, banging on doors and growling, from highsun till dusk. Home and shop owners are expected to give the children candy, fruits, or small items. Those who give no treat can expect to become the target of a trick at sundown. This mischief typically takes the form of “troll scratchings” at doors and windows. Those with more malicious intent sing screechingly in the wee hours, and hurl raw eggs at windows, signs, and the heads of those who try to stop them. Have some candy on hand or some sweet rolls, and all will be calm where you live.
Liar's Night (Volo's Waterdeep Enchiridion, page 25)
Liar's Night was a holiday celebrated in Waterdeep on the 30th day of
Marpenoth... Liar's Night was celebrated by donning costumes or
glamers to disguise one's self and pretend to be that which they were
not... The celebrations began in the evening when people placed
candles into gutted pumpkins or gourds that had faces carved into
them... Tricks, pranks, and even pickpocketing were common on this
night; thus tradition had people filling their pockets with candy in
lieu of money.