You can try using ZQB - Zanthor's Quest Broadcaster (Beta)
Or alternatively use a quest complete macro on them all at the same time.
I find if I turn in quests and one dings before the rest they will still get the extra xp because they all handed in simultaneously.

also, if you're killing mobs which are red to a character, they will get very little if any XP compared to killing an orange/yellow/green mob.
Ideally kill yellow level mobs.