I'm using:

#showtooltip Serpent Sting
/assist [target=party1]
/petattack
/castsequence reset=3/combat ,Serpent Sting

Basically, the first press sends the pets in.
The second keypress casts Serpent Sting, which starts auto attack.

That way, you can have the pet on the way to the target, prior to taking threat yourself.

*EDIT*
The Leader does not have the assist line.
I use a script to put the current leader on Bar 2, with the slaves on Bar 1.
Bar 2 is identical to Bar 1, except the abilities do not assist party1 (which is toon #2 for the leader)