The Necromancy School's Grim Harvest ability restores HP when you kill an enemy with a spell of 1st-level or higher. Does this apply when minions created by a spell, such as Animate Dead, or Conjure Elementals, kill an enemy?
Grim Harvest ...you gain the ability to reap life energy from creatures you kill with your spells. (PHB p.118 , emphases mine)
In the case of Animate Dead, the spell that you cast "imbues your target with a foul mimicry of life," and then that target may go on to kill something. Likewise with Conjure Elemental with the spell "you call forth an elemental servant." In each case your spell does something, then another creature kills.
You do use your bonus action or words to command the undead or elemental, respectively, to do things, but it's still the target of your command doing the killing, not you.