Get a battle animation for each pet, then make a short animation of the pet attacking. Create varibles for exp, and exp to level (unless you don't want them to level.) Then make a battle command for each character with a pet, make it activate whenever they use a certain command (it can be only one of them, or all of them, for example; you could have the pet attack whenever the player attacks.) Now make a randomized variable that goes between 1 and the number of monsters in the battle.
Then make a branch for each number, in each of these you show the pet's attack animation on the appropriate monster, and then have it do damage (you can make an attack stat for the pet, so when it levels the damage becomes greater,) according to what you want, if you made an attack stat for your pet, set it to that.
This makes for a simple pet system, it won't show the pet when it's not attacking either. You can use variables, such as a 1-5 random varible for making the pet hit or miss. And you can randomize a bonus, or penalty to the pet's attack stat, so it doesn't always do the same amount of damage. You can also randomize the use of a skill once they are a certain level. There really are many possibilities.
I hope I helped.