Have the sim active, click them, and it's under Magic. If I remember right, it's NOT under spells, I believe it's just under Magic.
You do have to be careful early on, a newbie spellcaster with no bloodline will really build charge fast. As you level up and take more perks, you build a lot less, and by the time you max you won't have to worry about it anymore. And then their child grows up and starts learning, and you've forgotten that that's a thing, and they blow themselves up... Good thing it doesn't kill you the first time!