I do this all the time with my Warlocks and their pets. When you reach the point that killing with the pet is too slow, then the pet becomes support instead of the main killing machine. I often use 2 level 70 Warlock pets. so the time is extended before killing with them alone is too slow.

On the other hand, slower killing is not really all that bad. While the pet is taking down a mob, this gives you time to scout out the next target, loot, pick flowers, pet your dog, pee, etc.

Assuming your Warlock or Hunter is level 70, when your lowbies get to the Outlands and start killing 62 level and above (i.e. green to the level 70), you can begin using the Warlock's spell power to help in the fights, and still get full XP for your lowbie group.

Also when you reach the point of taking on 62 level mobs, you can opt to help only one lowbie instead of 2 or more grouped. Any level 70 can help at this point, its not about just using pets anymore. Its the same routine ... tag with lowbie, kill with level 70. HOWEVER, if you are using level 70 Warlocks or Hunters, DON'T use their pets to help just one lowbie as that will reduce the amount of XP gained. Yeah, odd.