There are children, aged 2-years-old and up, who enjoys dressing up and knocking on every door in the neighbourhood. But your child's understanding of the concept of Halloween actually depends on how well you have made him understand that it is just an act. For some children, seeing someone dressed in scary attire may frighten them, but there are those who would just laugh at it.
Take note that young children may not distinguish what is real and what is not, and seeing their Mummy or someone from the neighbourhood dressed in a scary outfit may not be fun at all. But once your child grasps the concept of why people are wearing scary costumes during Halloween, trick-or-treat will also have a great appeal on them.
