My audience would probably be comprised of fifth to tenth grade students.
Books are the way to go for your situation. Buying downloads of single effects or props for single effects isn't going to be cost effective. Each effect runs $8 to $25 or more. Getting a book you reduce the cost per effect to $1 or less. In your situation, you will want to have a number of effects to perform.
Start with Mark Wilson's book ($15). That will give you a broad range of effects. It has everything including cards, coins, bills, rope, sponge balls, billiard balls, mental magic and stage illusions. That will give you a grounding in the basics. There are several effects from that book in my show -- that says something since I have over 150 other books to choose effects from. My only complaints is that the sleight of hand section for cards isn't that great and the fact that Mark likes to point at the hand where the prop (coin, sponge ball, billiard ball, etc) isn't.
I think Scarne on Card Tricks ($10) is a great investment for the price. There is a ton of self-working material in there that can get you performing quickly while you learn sleight of hand. If you got one book to do card tricks, this would be it.
If you want to learn sleight of hand for cards, get Card College Volumes 1 and 2 (around $35 each). Those two volumes are designed to be a complete course in the basics) . Alternatively, you can pick up Royal Road to Card Magic ($12) and Expert Card Technique ($15). Card College is more expensive but is easier to work through. Royal Road and ECT are from the 1940s and 1950s so the presentation is dated but the techniques are solid. If you like Scarne on Card Tricks, pick up Encyclopedia of Card Tricks ($12) - it has self working material plus some great work with gaff cards like double backers, double facers, short cards, etc.
If you want to learn coin magic, get Bobo's Modern Coin Magic ($9) It is inexpensive and well worth it.
I think your best resource beyond the basics will be Paul Harris' Art of Astonishment book series (around $35). That has strong close-up magic that will impress kids of those ages but the catch is that it is more intermediate material. But as the title suggests, this is the stuff of real astonishment.
Take your time. Start with Mark Wilson's book and Scarne on Card Tricks. If this is more than a passing fancy, then pick up a couple more card books and Bobo. When you feel comfortable with that, pick up Art of Astonishment.