No, they do not need to be level 100 to get Volt Tackle. Here is how to get Volt Tackle:
Have a Pikachu/Raichu (female) in Daycare holding the Light Ball. Just have any other partner in the same group as your Pikachu/Raichu for the second Pokemon.
Wait until they lay an egg (that must have been tough on Pikachu) and then hatch the egg. The Pichu will have Volt Tackle.