I believe that both the father and the mother will need to know Heal Bell in order for the baby Miltank to know Heal Bell upon hatching. If you are trying to pass on an egg move or a TM move, then only the father needs to know the move. If you are trying to pass on a move to a newborn Pokémon that it normally will learn on it's own at a higher level, then both parents need to know the move.