In Permafrost (Level 90 Raid): Part of the "Lady Vox's Lair" raid event (see this quest entry for event and loot information).
In the Heart of Fear: Part of the mini raid event "Monarch Widow" (see this quest entry for event and loot information).
In Newbie Zones: The spiderlings are the non-posionous version of the spiders. They are non-aggressive, but EXTREMELY social and will join in if you're fighting one anywhere near them. While they can be excellent xp for the younger player, they do come in a variety of level ranges. It is very important for the newer player to /consider the spiders closely to ensure they're of a reasonable level before taking them on.