I have seen so many threads about training low level pets i am starting to wonder if there should be a sticky about it , not just for level 70,s but all levels.
At all levels it is better to find caster mobs if at all possible .
Always check out Petopia first as the pet you want may be available with the same skin at a far better level .
Remember always to attack the lowest mobs possible that are still green to you , the pet will get less damage and the same for you if they decide to attack you ,
Always attack from maximum distance , that means your threat takes far longer to build , and yes i have tried that out many times so i know it is right.
AT a very low level of pet to you the pet mending will be the most important thing but you can always keep the pet on passive or use cower , which is about the only time i can think of to use cower , i prefer to send in the pet and have tried out low level pets and either make sure it is casters or be prepared to melee as you will take aggro from your pet.
If the pet was very low like a level 3 boar you would find the pet will level almost every kill for the first 10 to 20 levels from level 20 to 30 i found it was more like every few kills , every 10 levels it gets slower and slower but is not bad up to 60 , from 60 to 70 , WOW that does get boring but most pets will be able to hold aggro if you take just auto shots .
At all levels of Player i found if the pet is about 5 to 10 levels below you it should hold aggro , If you still take aggro stand at max distance and be prepared to stop shooting until pet regains aggro ,
An add-on like omen will help you a great deal in understanding threat from your shots ,
it seems the hunters way is to always want to try new pets out , i know i have ,there is not a family of pets i have not tried as i rose to 70 , i have spent hours grinding so my pets gain levels , but i am a farmer at heart anyway and the quests are secondary to me.