The moves obtained by a Pokémon when hatching are as follows:
By default, the baby Pokémon will start with any moves that it learns at level 1. (In Generation II and Generation III, they knew all moves that are level 5 and below in their learnset.)
If both parents know a move that the baby can learn via leveling up, the Pokémon will inherit that move.
Prior to Generation VI, if the father knows any TM or HM moves (and in Crystal, Move Tutor moves) that the baby is also compatible with, it will inherit the move. If a genderless Pokémon breeds with Ditto, the resultant baby will inherit any compatible TM or HM moves that the genderless non-Ditto Pokémon parent knows.
Any passable moves the mother knows now take priority over the father's moves.
If either parent (or, prior to Generation VI, the father) knows any specific Egg moves that the baby can learn, the baby will inherit the move.
From Pokémon Emerald onward, for a baby Pichu, if either parent is holding a Light Ball, the Pichu will know Volt Tackle. Volt Tackle cannot be passed down as a normal Egg move.
If this would result in the baby possessing more than four moves, the moves take priority in the order listed: Volt Tackle, mother's Egg moves, father's Egg moves, TM and HM moves, inherited level-up moves, baby's default moveset.
