These lovely girls practice an old tradition. Trick or treating begun in the Middle-Ages.
Children and sometimes poor adults would dress up in costumes and go from door to door during the next day
after Halloween festival begging for food or money. Children in costumes would go from house-to-house, asking for treats with the phrase "Trick or treat".
The "treat" is usually some candies, The "trick" refers to the threat to perform a mischief on the homeowners if no treat is given.