Even though I agree with what you're saying, moderation is something that is hard to achieve if you're already out of control. Moderation is hard to achieve unless you have a concrete goal. Moderation is hard to achieve if you're a young child.
I am not following this logic. Of course moderation is hard to achieve if you're a young child - that's why children have parents.
(I use "you'" in the hypothetical sense below.)
If you are the parent, be the parent. You can tell your child no. You will survive if your child gets angry at you and acts out. You'll even survive the oh-so-dreadful embarassment if he does so in public place. You can manage through the inconvenience caused by actually having to attend your child instead of handing over a gadget to entertain him. You do not have to give him what he wants - because you are the parent.
Yes, by all means, explain WHY - always explain why.. Even at 2 or 3 yrs, children understand a lot more than most people think they do. But don't lose sight of the fact that it's not a negotiation. Explain why *after* your expectation is met. Discuss it *after* the behavior has stopped.
You are not your child's friend, you are his parent. Friendship can come later, if he survives through is twenties long enough to grow into a reasonable facsimile of a human being.