harry potter - Why were the Death Eaters so lenient during the fight at the Department of Mysteries?

This always struck me as odd. When Harry, Ron, Hermione, Ginny, Neville and Luna all go to the Department of Mysteries and are confronted by the Death Eaters, why are the Death Eaters so lenient with them? When the actual fighting starts, Lucius says to kill the kids as necessary (except Harry).

"Harry put his ear close to the door to listen and heard Lucius Malfoy roar: "Leave Nott, leave him, I say, the Dark Lord will not care for Nott's injuries as much as losing that prophecy - Jugson, come back here, we need to organize! We'll split into pairs and search, and don't forget, be gentle with Potter until we've got the prophecy, you can kill the others if necessary - Bellatrix, Rodulphos, you take the left, Crabbe, Rabastan, go right - Jugson, Dolohov, the door straight ahead - McNair and Avery, through here - Rookwood, over there - Mulciber, come with me!"

_{Order of the Phoenix - page 788 - Bloomsbury - chapter 35, Beyond the Veil}

But the Death Eaters proceed to use spells that are not only non-fatal, but downright silly. There are about four instances in which the spells are apparently fatal, but most of the spells they use are strange for a hardened bad guy to use.

"And what about Ron?" said Harry fearfully, as Ron continued to giggle, still hanging off the front of Harry's robes.
"I don't know what they hit him with," said Luna sadly, "but he's gone a bit funny, I could hardly get him along at all...."

_{Order of the Phoenix - page 796 - Bloomsbury - chapter 35, Beyond the Veil}

"Harry, it'll suffocate him!" screamed Ginny, immobilized by her broken ankle on the floor - then a jet of red light flew from one of the Death Eater's wands and hit her squarely in the face. She keeled over sideways and lay there unconscious.

_{Order of the Phoenix - page 798 - Bloomsbury - chapter 35, Beyond the Veil}

But nothing happened - one of the Death Eaters shot their own Stunning Spell at Neville; it missed him by inches.

_{Order of the Phoenix - page 798 - Bloomsbury - chapter 35, Beyond the Veil}

Its owner was lying on his side, bleeding from the head, and his attacker was now bearing down upon Harry and Neville: Dolohov, his long pale face twisted with glee.
"Tarantallegra!" he shouted, his wand pointing at Neville, whose legs went immediately into a kind of frenzied tap dance, unbalancing him and causing him to fall to the floor again. "Now, Potter -"

_{Order of the Phoenix - page 802 - Bloomsbury - chapter 35, Beyond the Veil}

They mostly try to stun the others, which is more understandable (although still pretty tame), but I find it makes the Death Eaters a lot less intimidating when they use something like Tarantallegra, which Malfoy used on Neville in their first year. I know JK Rowling didn't want to kill off the characters, but I find the fact that they didn't kill the kids very weird, especially as said kids proved themselves to be surprisingly good fighters.

Answer

This question goes to a more fundamental aspect of the nature of combat. In combat, even doing the wrong thing immediately is of more use than doing the correct thing too slowly. Statistics about gun fights show that even at distances as close as 10 feet, more than half of all shots miss.

We have to presume, then, that there is something about the nature of a fast paced free-for-all such as what happened at the ministry that caused the use of such spells, rather than Avada Kedavra.

The most reasonable theory is that most wizards in these kinds of situations have one or more "go-to" spells that they instinctively use. Harry, as we know from the escape sequence in the beginning of The Deathly Hallows, is known for his signature use of Expelliarmus. Presumably, the idea with such spells is to survive and incapacitate your opponent so that you can properly deal with them later.