As a kid, my dad would bring me to this magical store filled with everything from doll eyes to giant magnets. It's still one of my favourite Toronto places (though I miss the sprawling main-floor location) and as a teacher, I send curious kids there whenever possible.
The staff are particularly helpful to beginners putting together their first circuits. Please ask if you're at all confused. I've never encountered anyone dismissive or rude here.
The only downside us that you can't always find exactly what you're looking for, though I suppose that is part of the appeal.