As @EugeneT says, demand is a very strong word; generally it would only be used in a negotiation between neutral-to-hostile parties. (When two warring countries sit down to negotiate a cease-fire, for example, they might each bring a list of things they will demand as part of the settlement.)
If you are making a demand, generally there is an implied (or explicit) ultimatum behind it: "You give me what I demand, or else I will cause some unpleasant consequence!"
So you probably want to use a friendlier phrasing most of the time, and especially in this situation. Some suggestions:
I need to get a copy of the birthday video from you.
Can you send me a copy of the birthday video?
Please send me a copy of the birthday video.