In terms of tackling, you must take them through one-on-one contact drills such as the one Webby suggested. One-on-one drills are very important and much better than using a tackle bag. As a referee, I hate seeng players fly at others as if at a tackle bag. In junior levels it is downright dangerous and completely unnecessary. More to the point tackle bags don't teach technique. Don't use pads use one-on-one contact and familiarise them with the good tackle position - generally around the waist, or a little lower with the shoulder driving through.