I know it's not true in general, that's why I said it's an oversimplification. Given we're explaining vector calculus to a five year old, though, I thought it would be OK. What is true, though, is that if a vector field only has a single component then the curl does have to be perpendicular to the field. That's what's going on with linearly polarized light, and why I didn't feel too bad about oversimplifying.