The way calculus is usually taught is a mess. A mix of epsilon-delta formalism, without adequate motivation, differentials and excessive focus on computations. For young students, a great introductory textbook is Calculus Made Easy . It is around 100 years old, and develops all the material using infinitesimals. Which is essentially modern non-standard analysis, minus rigor. It is also the way Newton and Leibniz thou…
Develop the intuition, then crystallize and formalize the idea with a proof. Otherwise, it's just not going to make any sense.