I don’t recall what made it a Christmas special, but this aired during Christmas break in the mid to late ’70s in a time slot that had featured more typical kids’ Christmas fare on other days.
It was in colour with quite good production values, and I saw it on CBC or CTV (Canadian networks).
A boy discovers a UFO landed on a hill in the woods near his home. In the scene I remember, he somehow ends up inside the spaceship, trapped in a chamber. A weapon-like clear plastic L-shaped tube descends from the ceiling and turns ominously towards him. The boy’s fear turns to delight, though, as the tube starts shooting M&Ms or (Canadian) Smarties towards him, eventually filling the chamber to his waist.