Samhain is the old Celtic festival of the beginning of winter. At this time, the ancient Irish celebrated their liberation from oppressive conquerors (Robin Herne, 2004). Later, it came to mean the night when the veil between the worlds was thin and the living could commune with the dead. It also symbolises the Crone aspect of the Triple Goddess.
The Crone Goddess is particularly associated with Samhain in contemporary Paganism, because she is the midwife and layer-out, the one who cuts the cord of both life and death. She represents merciful release; but she also possesses the wisdom of old age. Wisdom is traditionally represented (in the Bible and in other traditions) as a feminine being or quality. Wisdom is the joining together of instinct and experience and knowledge. It is the wisdom of the body, the knowledge of when to act and when to refrain from acting, when to speak and when to keep silent. Wisdom comes from reflection upon experience and knowledge.
Other festivals at this time of year are the Mexican Day of the Dead (Dia de los Muertos) when people visit the graves of their loved ones and hold feasts in their honour; and the Christian festivals of All Saints and All Souls (also known as All Hallows, which is where we get the name Hallowe'en, a shortened form of the Eve of All Hallows).
Contrary to popular belief, the Hallowe'en custom of trick or treat did not originate in America. It has its origins in much older customs from the North of England called guising, where people would dress up as scary monsters and frighten their neighbours. The function of this custom is to express the repressed shadow side of the psyche in a harmless and fun way, so that we fear it less by making it visible.
Pagans build altars for their deceased loved ones, and recall them to mind in ritual at this time of year.
Many Unitarian and UU churches now celebrate Samhain and/or All Souls.
Several websites have suggestions for Samhain activities for children (but adults will enjoy them too): here's one with suggestions for children, by Akasha.
You can find out more about the ancient Celtic festival of Samhain from Robin Herne's article, Samhain Myths.
There's a lovely UU Samhain service by Elizabeth A Lerner.