As far as I know, you can only have ONE familiar summoned at any given time.
I, at least, have never had more than one present at any given time, so for me it isn't really an issue.
One per caster. Considering you can make casters as long as you have essence, and you all four of your children are natural casters, and all summons are independent units, you can have....7? 8? Or more Vigilant Minions out, in one army, dealing tons of unavoidable, attack all enemies damage. When they get leveled, they can cast it as much as three times per battle, each.
You do the math. I doubt they're intended to be that strong. They were just added without the necessary balancing/restrictions in place.